Guard spec.convert() against throwing

[DmitrySharabin]

spec.parse() calls are now guarded with try/catch (#129), but spec.convert() can also throw on values that parsed successfully but aren't valid input for the conversion step.

We should apply the same warn-and-degrade pattern to spec.convert() calls.

Should we degrade/fall back to the parsed value (that is correct on that step)?

[LeaVerou]

Yes, but we shouldn't fall back to the parsed value, since it could be a completely different type, we should probably fall back to the previous value