Fix warnings related to catch all exceptions

This is a minor change on my way to warming up to uPickle!

Without this fix `sbt test` produces a couple of warnings of the following kind:
```
[warn] /Users/ramnivas/external-dev/upickle/jvm/shared/test/scala/upickle/FailureTests.scala:66: This catches all Throwables. If this is really intended, use `case e : Throwable` to clear this warning.
[warn]           case e =>
[warn]                ^
```