Relax requirements for generic conversion to TestCaseError

[udoprog]

This permits using alternative error types which doesn't implement std::error::Error, like failure::Error directly in proptest blocks.

This is accomplished by relaxing the requirements on the blanket conversion mechanism to only require fmt::Display. Unfortunately this would cause TestCaseError to conflict with the blanket implementation, so it had to be reworked.

[AltSysrq]

The breaking change is pretty unfortunate, though this otherwise looks like a good thing to have. For now, I'm going to keep this open under consideration for the next time we have a breaking release.