The return value of bl2_plat_handle_bl30() used to be ignored.
This patch modifies the function load_bl30() so that it now
checks this return value and returns it to bl2_main().

This patch also unifies the error handling code across the
load_blx() functions so that they return a status code in all
cases and bl2_main() has the sole responsibility of panicking
if appropriate.

Change-Id: I2b26cdf65afa443b48c7da1fa7da8db956071bfb