Display a dialog in Pidgin if purple_core_init returned FALSE

This also adds an environment variable so developers (or users for some reason)
can set a custom error message for purple_core_init to return. The intent is to
allow UI developers to control the message and make sure things display as they
expect them to.

Testing Done:
used the `PURPLE3_CORE_ERROR_MESSAGE` environment variable to test the dialog including one with multiple lines of `a`'s with no breaks and everything displayed fine.

Reviewed at https://reviews.imfreedom.org/r/2007/