[gdb/symtab] Add gdb/dwarf2/error.h

Add a new header file gdb/dwarf2/error.h, containing macros:
- DWARF_ERROR, and
- DWARF_ERROR_PREFIX.

The DWARF_ERROR_PREFIX is to be used in dwarf errors in a follow-up patch.

No functional changes.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>