Use e.to_s as fallback for e.message

Some exception have nil for it's message, and it makes it unserializable

In this case, you can see exception like below
```
ArgumentError: The property 'value' is required for this Dash.
	from .../ruby/2.0.0/gems/hashie-2.0.5/lib/hashie/dash.rb:165:in `assert_property_required!'
	from .../ruby/2.0.0/gems/hashie-2.0.5/lib/hashie/dash.rb:125:in `[]='
	from (eval):6:in `value='
	from .../ruby/2.0.0/gems/sentry-raven-0.7.1/lib/raven/event.rb:194:in `block in parse_exception'
	from .../ruby/2.0.0/gems/sentry-raven-0.7.1/lib/raven/interfaces.rb:11:in `call'
	from .../ruby/2.0.0/gems/sentry-raven-0.7.1/lib/raven/interfaces.rb:11:in `initialize'
	from .../ruby/2.0.0/gems/sentry-raven-0.7.1/lib/raven/event.rb:88:in `new'
	from .../ruby/2.0.0/gems/sentry-raven-0.7.1/lib/raven/event.rb:88:in `interface'
	from .../ruby/2.0.0/gems/sentry-raven-0.7.1/lib/raven/event.rb:192:in `parse_exception'
	from .../ruby/2.0.0/gems/sentry-raven-0.7.1/lib/raven/event.rb:142:in `block in from_exception'
	from .../ruby/2.0.0/gems/sentry-raven-0.7.1/lib/raven/event.rb:59:in `call'
	from .../ruby/2.0.0/gems/sentry-raven-0.7.1/lib/raven/event.rb:59:in `initialize'
	from .../ruby/2.0.0/gems/sentry-raven-0.7.1/lib/raven/event.rb:139:in `new'
	from .../ruby/2.0.0/gems/sentry-raven-0.7.1/lib/raven/event.rb:139:in `from_exception'
	from .../ruby/2.0.0/gems/sentry-raven-0.7.1/lib/raven.rb:108:in `capture_exception'
```