gdb/python: test exception case for gdb.solib_name

The gdb.solib_name() and Progspace.solib_name() functions can throw an
exception if the address argument is not a valid address, but this is
not currently tested.

This commit adds a couple of tests to check that exceptions are thrown
correctly.

An early version of this commit updated the documentation, but it was
pointed out that lots of functions throw an exception if passed an
argument of the wrong type, and we don't document all of these, it's
kind-of assumed that passing an object of the incorrect type might
result in an exception, so this updated version leaves the docs alone,
but I do think adding the extra tests has value.

There's no changes to GDB itself in this commit.

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