FOAM_ABORT pre-empts throwing exceptions

As noted prior to release by @andy, running with FOAM_ABORT means that failed loading of function objects translates into a real failure instead of being generous and emitting a warning. With issue #1779 (closed), it looks like time to make a minor adjustment to the error class.

I think that if code has requested throwExceptions() this should be honoured by the internals of error. Ie, swap the order of if/else if in error.C around line 240.

Opinions, objections? @andy and @Mattijs