[AD] Error reporting from Allegro functions

[SiegeLord <slabode@xxxxxxxxxx>]

Allegro currently provides two separate methods of indicating the 
precise cause of an error (usually a NULL return value). The most 
common method right now uses the ALLEGRO_WARN/ERROR macros. The other 
method uses the al_get_errno/al_set_errno functions which are basically 
unused. Some places, like the memfile addon, use al_set_errno with 
great regularity, and some places, like the image addon, provide very 
detailed log messages (for most errors... ironically not for the most 
common one of non-existent file).

So my question is... which method is preferred and to what extent? 
Should every "return NULL;" be accompanied with a logging macro? Should 
every malloc be accompanied with al_set_errno(ENOMEM); ?

Although the trace macros are attractive, I find them to be problematic 
as they are only enabled in debug mode. While the windows binaries we 
supply have the debugging versions included, as far as I can tell 
neither the Fedora packages nor the eventual Debian packages will have 
this feature. Perhaps we should move away from these macros? Also 
non-log error reporting is more attractive from the point of view of 
wrappers of Allegro.

If we do move away from macros, perhaps we should add a 
al_set_error_string function if al_set_errno is deemed too 
coarse-grained.

Anyway, just some thoughts I had after reading this thread: 
https://www.allegro.cc/forums/thread/612095

-SL