Improved Cone Search time-out error message

As discussed in #2687 .

Also updated an unrelated test because it looks like STScI VAO pruned some services off their registry. This has no impact on the `astropy` side of things, so I didn't bother to mention it in change log.
@pllim - can you revert these changes? They apply to the 1.0 section which will not be there in the 0.4.x branch, so this would cause issues when backporting.