Show an error when a level is unsuitable for sharing

[thejoshwolfe]

These conditions should make the starting state for a level invalid:

• collision violation: an object is on a tile that is not air-like.
• gravity violation: an object will fall if anything happens to advance time.
• no exit.
• no snakes.
• number of portals is not 0 or 2.

Checking if the level is solvable is not within this scope of this issue.

[XeroOl]

If you embed the solution into the URL, the program could warn you if the solution doesn't work or isn't in the URL