More Reader/Writer coordination errors

These are similar in nature to Issue #103 (closed):

1. When a Reader object opens an image with color type 3 and a method like asRGB is called, it adds info["colormap"] = False, but doesn't touch info["palette"] at all. Meanwhile, Writer(**info) ignores colormap entirely and just checks if it received a palette. This results in an error either in the constructor (if there's an alpha channel) or in the write method (each row has more values than expected).
2. When a Reader object translates a tRNS chunk to an alpha channel, it adds info["alpha"] = True, but doesn't clear info["transparent"]. Writer(**info) will raise an exception as a result.

Example session in interactive mode to demonstrate these issues: shell_session.py (Note: To avoid potentially doxxing myself, i did edit this slightly. It should still be valid, but i haven't re-tested, so you should verify on your end.)

(Also worth noting is that the errors printed the exception type twice.)