Add error handling to 'Change source' form field

[geografa]

When changing sources in Studio, if a sources have a space in the comma-separated list, no error is thrown.

• [ ] add tip on usage to copy below the form
• [ ] add error handling if a space is used

[geografa]

Hmm.. looks like this is in place. Testing this now on other browsers.

[ajashton]

See #805 - that type of warning is not supported by atom shell.