The tests for error scenarios were done by manually checking
error strings. Improved them by checking the actual error type
instead of just the string.

Printing the actual error in case of failure instead of a
generic string.

Also added a new scenario with both an invalid byte and an
invalid length string to verify that the length is checked first
before doing any computation.

Change-Id: Ic2a19a6d6058912632d597590186ee2d8348cb45
Reviewed-on: https://go-review.googlesource.com/55256Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>