Failed decoder should throw decoding-specific error (not ValueError)

[Johan Rosenkilde]

There should be a new exception class DecodingFailure. This should be thrown when a decoder fails, possibly with a message string (if there is information in how it failed. If it is just "too many errors" and nothing else, then no message), and later on possibly subclassed.

[David Lucas]

Resolved issue #38

→ <<cset b8354e44eccc>>