Improve error messages for invalid content (#377)

Previously we returned an HTTP 500 "error canonicalizing entry" error if
Rekor was unable to parse or verify the proposed content of a new log
entry. This adds a new error type ValidationError that allows
implementers of the Canonicalize method to delineate between internal,
transient errors and errors that clients can rectify.

With this patch, errors parsing or validating (provided or referenced)
artifacts will return an HTTP 400 message to the client with a message
about the issue.

Fixes: #362

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>