fix warnings on try-catch

This commit removes the following warnings:

[warn] /Users/mikeg/workspace/scalacheck/src/main/scala/org/scalacheck/Prop.scala:404: This catches all Throwables. If this is really intended, use `case e : Throwable` to clear this warning.
[warn]     try { p: Prop } catch { case e => exception(e) }
[warn]                                  ^
[warn] /Users/mikeg/workspace/scalacheck/src/main/scala/org/scalacheck/Test.scala:203: This catches all Throwables. If this is really intended, use `case e : Throwable` to clear this warning.
[warn]     try { Left(x) } catch { case e => Right(e) }