gdb/testsuite: handle errors better in test_compiler_info

gdb/testsuite: handle errors better in test_compiler_info

Now that get_compiler_info might actually fail (if the language is not
handled), then we should try to handle this failure better in
test_compiler_info.

After this commit, if get_compiler_info fails then we will return a
suitable result depending on how the user called test_compiler_info.

If the user does something like:

  set version [test_compiler_info "" "unknown-language"]

Then test_compiler_info will return an empty string.  My assumption is
that the user will be trying to match 'version' against something, and
the empty string hopefully will not match.

If the user does something like:

  if { [test_compiler_info "some_pattern" "unknown-language"] } {
    ....
  }

Then test_compiler_info will return false which seems the obvious
choice.

There should be no change in the test results after this commit.