[gdb/symtab] Use DWARF_ERROR_PREFIX

Result of:
...
$ sed -i 's/"Dwarf Error: /DWARF_ERROR_PREFIX\n"/' gdb/dwarf2/*
...
and manually fixing indentation.

No functional changes.

Tested on x86_64-linux.

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