In gutils/gimagereadbmp.c, return error message if can't open/read file
In gutils/gimagereadbmp.c, getshort() now checks for file read errors.
In gutils/gimagereadbmp.c, exit if error reading bmp header info.

Added error check for getlong(). Changed fillbmpheader() to -1 on error.
In gutils/gimagereadbmp.c, move memory calls into GImageRead_Bmp()

Replaced galloc() with malloc() and gfree() with free().
Moved memory creation functions here so that error messages can be given
based on either lack of memory or due to file read errors.
In gutils/gimagereadbmp.c, cleanup if read error. gfree() now free().
In gutils/gimagereadbmp.c, missed these in copy-n-paste (of last 5 commits)