[gdb] Eliminate catch(...) in get_test_insn

In get_test_insn in gdb/disasm-selftests.c, we find this code:
...
            try
              {
                kind = gdbarch_breakpoint_kind_from_pc (gdbarch, &pc);
                insn = gdbarch_sw_breakpoint_from_kind (gdbarch, kind, &bplen);
              }
            catch (...)
              {
                continue;
              }
...

The catch is there to catch memory errors, but it swallows all exceptions, including
gdb_exception_quit and gdb_exception_forced_quit.

Fix this by limiting the catch to gdb_exception_error.

Tested on x86_64-linux, by rebuilding and running gdb.gdb/unittest.exp.

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