Only throw Exceptions, not Throwables

[whilo]

According to the JVM specification Throwables should not be caught, because they can include runtime Errors that are not recoverable https://docs.oracle.com/javase/specs/jls/se7/html/jls-11.html. Slingshot currently casts ExceptionInfo to Throwable https://github.com/scgilardi/slingshot/blob/release/src/slingshot/support.clj#L37, which forces me to catch Throwables including severe unrecoverable runtime errors. It should be sufficient to put a cast to ^Exception there. I can open a PR if this sounds reasonable.