gdb: make the error message for unreadable objfiles more informative

gdb: make the error message for unreadable objfiles more informative

When GDB is unable to read an objfile, it prints the error message "I'm
sorry Dave, I can't do that. Symbol format `%s' unknown.". While it is a
great reference, an end user won't have much information about the
problem.

So far this wasn't much of a problem, as it is very uncommon for GDB to
be unable to read an objfile. However, a future patch will allow users
to selectively disable support to some formats, making it somewhat
expected that the message will be seen by end users.

This commit makes the end message more informative and direct.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=13299
Approved-By: Tom Tromey <tom@tromey.com>