Simplify the error subsystem, get rid of error2.[ch].

Simplify the error subsystem, get rid of error2.[ch].

This commit removes error2.c and the surrounding infrastructure of
the build system, getting rid of ~600 LOC.

After the change there are no more subsystems for error codes, and we
don't need to host-compile error2.c any more. Since all executables
now contain the text of every error code, the change has some impact
on the sizes of the (stripped) executables:

	     Before:                         After:

	 64K para_afh             	 71K para_afh
	 43K para_audioc          	 47K para_audioc
	256K para_audiod          	259K para_audiod
	 64K para_client          	 71K para_client
	 39K para_fade            	 47K para_fade
	141K para_filter          	144K para_filter
	 51K para_gui             	 59K para_gui
	252K para_play            	255K para_play
	 97K para_recv            	104K para_recv
	227K para_server          	230K para_server
	 60K para_write           	 67K para_write

This increase in size is justified by the major simplification.