Do not fail silently if IP in connection window is invalid

Follow up #1938 by @softins

If an invalid IP is entered, the connection window closes after clicking on connect and Jamulus does not give an error message.

Proposed behaviour:

1. Check IP/Domain on entry and give a red warning "The IP you entered is invalid. Make sure you enter it in a correct format"
2. Same as above, but after clicking on connect as error message

Start by reproducing the invalid-IP behavior in Jamulus's connection window, then trace what happens when an invalid IP or domain is entered and when Connect is clicked. Done means invalid input produces the specified red warning on entry and an error message after connecting, without silently closing the window.

Written by the indexing model from the issue text.