Invalid "cheat codes" are not rejected and never timeout

[crwood]

Gridsync supports "cheat codes" as a means of (optionally) embedding the credentials needed to connect to a storage grid into the application bundle itself while still emulating the UX of a "real" magic-wormhole code during initial setup. Because cheat codes are intended to bypass the actual magic-wormhole functionality, however, and do not open up a real TCP connection, the standard connection timeout logic is never applied, thus leaving a user who incorrectly enters a 0-prefixed cheat code to wait indefinitely for a "connection" that will never complete.

Instead, Gridsync should check for and reject any/all 0-prefixed codes which to not correspond to valid cheat codes, informing the user immediately that the code is invalid -- and thereby preventing needless waiting and frustration in the event of a typo or incorrect cheat code.

[crwood]

Fixed via #186