app: gimp_brush_load_brush(): add special error for possible old files

The obsolete .gbp format had a 3-byte pattern following a 1-byte
brush, when embedded in a brush pipe, the current code tries to load
that pattern as a brush, and encounters the '3' in the header. Detect
that and suggest to re-save the file because the plug-in still loads
the legacy format.

(cherry picked from commit 83bd1c86faa293b1c6456147395be18ade24f748)