Stop shadowing the word `error` because we want to know what the
specific exception message was.

Also give more details on exactly which value failed. We don't know who
submitted the value, but perhaps it's unique enough that we can debug it better.

Fix #1318

Bug 1:
When `value` is None, trying to call `len(None)` throws an exception.

Bug 2:
When len(`value`) <= 100, the code currently prints b'' rather than
`value`.