Polish all error messages

[dtolnay]

The current error messages have an unfriendly Display representation. For example SerializeError in 9e47b71:

impl fmt::Display for SerializeError {
    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        match *self {
            SerializeError::IoError(ref err) => write!(f, "IoError: {}", err),
            SerializeError::Custom(ref s) => write!(f, "Custom Error {}", s),
            SerializeError::InfiniteSequence => write!(f, "InfiniteSequence"),
            SerializeError::SizeLimit => write!(f, "SizeLimit"),
        }
    }
}

Let's audit all the error messages in this crate and make sure everything has a self-explanatory message that a human can read and understand what went wrong.

[TyOverby]

I think a neat feature of the errors (once merged) would be tracking if they came from encoding or decoding.

Example:

While decoding, bincode hit an IoError from the underlying reader: $IOERRORTEXT
vs
While encoding, bincode hit an IoError from the underlying writer: $IOERRORTEXT

[dtolnay]

I filed individual issues for the error message problems I noticed.

[TyOverby]

Closed all of these (I think)