Throw an error message when $WORKON_HOME is not set

Took me a while to figure this one out, and I have a default installation of virtualenvwrapper -- this is a soft fix, just put an error message. But perhaps the fix should be to use the default value `~/.virtualenvs`.

plugins/virtualenvwrapper/virtualenvwrapper.plugin.zsh

 virtualenvwrapper='virtualenvwrapper.sh'
 if (( $+commands[$virtualenvwrapper] )); then
+
   source ${${virtualenvwrapper}:c}
 
-  if [[ ! $DISABLE_VENV_CD -eq 1 ]]; then
-    # Automatically activate Git projects's virtual environments based on the
-    # directory name of the project. Virtual environment name can be overridden
-    # by placing a .venv file in the project root with a virtualenv name in it
-    function workon_cwd {
+  if [[ "$WORKON_HOME" == "" ]]; then
+    echo "\$WORKON_HOME is not defined so ZSH plugin virtualenvwrapper will not work"
+  else
+
+    if [[ ! $DISABLE_VENV_CD -eq 1 ]]; then
+      # Automatically activate Git projects's virtual environments based on the
+      # directory name of the project. Virtual environment name can be overridden
+      # by placing a .venv file in the project root with a virtualenv name in it
+      function workon_cwd {
         if [ ! $WORKON_CWD ]; then
-            WORKON_CWD=1
-            # Check if this is a Git repo
-            PROJECT_ROOT=`git rev-parse --show-toplevel 2> /dev/null`
-            if (( $? != 0 )); then
-                PROJECT_ROOT="."
-            fi
-            # Check for virtualenv name override
-            if [[ -f "$PROJECT_ROOT/.venv" ]]; then
-                ENV_NAME=`cat "$PROJECT_ROOT/.venv"`
-            elif [[ -f "$PROJECT_ROOT/.venv/bin/activate" ]];then
-                ENV_NAME="$PROJECT_ROOT/.venv"
-            elif [[ "$PROJECT_ROOT" != "." ]]; then
-                ENV_NAME=`basename "$PROJECT_ROOT"`
-            else
-                ENV_NAME=""
-            fi
-            if [[ "$ENV_NAME" != "" ]]; then
-                # Activate the environment only if it is not already active
-                if [[ "$VIRTUAL_ENV" != "$WORKON_HOME/$ENV_NAME" ]]; then
-                    if [[ -e "$WORKON_HOME/$ENV_NAME/bin/activate" ]]; then
-                        workon "$ENV_NAME" && export CD_VIRTUAL_ENV="$ENV_NAME"
-                    elif [[ -e "$ENV_NAME/bin/activate" ]]; then
-                        source $ENV_NAME/bin/activate && export CD_VIRTUAL_ENV="$ENV_NAME"
-                    fi
-                fi
-            elif [ $CD_VIRTUAL_ENV ]; then
-                # We've just left the repo, deactivate the environment
-                # Note: this only happens if the virtualenv was activated automatically
-                deactivate && unset CD_VIRTUAL_ENV
+          WORKON_CWD=1
+          # Check if this is a Git repo
+          PROJECT_ROOT=`git rev-parse --show-toplevel 2> /dev/null`
+          if (( $? != 0 )); then
+            PROJECT_ROOT="."
+          fi
+          # Check for virtualenv name override
+          if [[ -f "$PROJECT_ROOT/.venv" ]]; then
+            ENV_NAME=`cat "$PROJECT_ROOT/.venv"`
+          elif [[ -f "$PROJECT_ROOT/.venv/bin/activate" ]];then
+            ENV_NAME="$PROJECT_ROOT/.venv"
+          elif [[ "$PROJECT_ROOT" != "." ]]; then
+            ENV_NAME=`basename "$PROJECT_ROOT"`
+          else
+            ENV_NAME=""
+          fi
+          if [[ "$ENV_NAME" != "" ]]; then
+            # Activate the environment only if it is not already active
+            if [[ "$VIRTUAL_ENV" != "$WORKON_HOME/$ENV_NAME" ]]; then
+              if [[ -e "$WORKON_HOME/$ENV_NAME/bin/activate" ]]; then
+                workon "$ENV_NAME" && export CD_VIRTUAL_ENV="$ENV_NAME"
+              elif [[ -e "$ENV_NAME/bin/activate" ]]; then
+                source $ENV_NAME/bin/activate && export CD_VIRTUAL_ENV="$ENV_NAME"
+              fi
             fi
-            unset PROJECT_ROOT
-            unset WORKON_CWD
+          elif [ $CD_VIRTUAL_ENV ]; then
+            # We've just left the repo, deactivate the environment
+            # Note: this only happens if the virtualenv was activated automatically
+            deactivate && unset CD_VIRTUAL_ENV
+          fi
+          unset PROJECT_ROOT
+          unset WORKON_CWD
         fi
-    }
+      }
 
-    # Append workon_cwd to the chpwd_functions array, so it will be called on cd
-    # http://zsh.sourceforge.net/Doc/Release/Functions.html
-    # TODO: replace with 'add-zsh-hook chpwd workon_cwd' when oh-my-zsh min version is raised above 4.3.4
-    if (( ${+chpwd_functions} )); then
+      # Append workon_cwd to the chpwd_functions array, so it will be called on cd
+      # http://zsh.sourceforge.net/Doc/Release/Functions.html
+      # TODO: replace with 'add-zsh-hook chpwd workon_cwd' when oh-my-zsh min version is raised above 4.3.4
+      if (( ${+chpwd_functions} )); then
         if (( $chpwd_functions[(I)workon_cwd] == 0 )); then
-            set -A chpwd_functions $chpwd_functions workon_cwd
+          set -A chpwd_functions $chpwd_functions workon_cwd
         fi
-    else
+      else
         set -A chpwd_functions workon_cwd
+      fi
     fi
   fi
 else