fromTryCatch shouldn't infer Null

[ceedubs]

If you don't explicitly provide a type parameter to fromTryCatch (on either Xor or Validated), the type parameter defaults to Null and it doesn't actually catch exceptions.

scala> Xor.fromTryCatch(throw new RuntimeException("blargh"))
java.lang.RuntimeException: blargh
  at $anonfun$1.apply(<console>:17)
  at $anonfun$1.apply(<console>:17)
  at cats.data.XorFunctions$FromTryCatchAux.apply(Xor.scala:229)
  ... 43 elided

scala> Xor.fromTryCatch
res1: cats.data.Xor.FromTryCatchAux[Null] = cats.data.XorFunctions$FromTryCatchAux@1039a2fc

scala> Xor.fromTryCatch[Throwable](throw new RuntimeException("blargh"))
res2: cats.data.Xor[Throwable,Nothing] = Left(java.lang.RuntimeException: blargh)

I think we should do two things here:

1. Add a helper to catch NonFatal exceptions, as that is probably generally what people want. This could be called fromTryCatchNonFatal or just catchNonFatal if the former is too verbose.
2. Make fromTryCatch take implicit evidence that the type parameter is not an unhelpful type such as Nothing (do we need to cover Null too?). Scalaz does this: https://github.com/scalaz/scalaz/blob/series/7.2.x/core/src/main/scala/scalaz/NotNothing.scala

[ceedubs]

Fixed in #592