Static JavaProxy#getJavaClass needs to clear $!

Calls from Ruby which go through the static `JavaProxy.getJavaClass` (ie. `field_writer`, `field_reader`, and `field_accessor`) can result in `$!` incorrectly containing a `NoMethodError` from its attempt to invoke `java_class`.  Ensure we clear it in the catch block.

(I stumbled on this because Minitest was loading all tests up but then executing none of them [because `'$!` wasn't clear](https://github.com/seattlerb/minitest/blob/master/lib/minitest.rb#L46))