[gdb/symtab] Change DWARF_ERROR from Dwarf Error to DWARF Error

[gdb/symtab] Change DWARF_ERROR from Dwarf Error to DWARF Error

It was suggested here [1] that the canonical prefix for dwarf errors
should not be "Dwarf Error: ", given that the canonical spelling is DWARF
instead of Dwarf.

Fix this by using "DWARF Error: " instead.

Given the use of DWARF_ERROR_PREFIX, that needs to be changed only in a single
location.

Tested on x86_64-linux.

Suggested-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>
[1] https://sourceware.org/pipermail/gdb-patches/2024-August/211258.html