Errors thrown by zipkin bubbles all the way to the top

[russell-matt]

When using zipkin-instrumentation-restify, an error that was being thrown in zipkin bubbled up all the way to the top, and out of the middleware, which causes the server to crash.

The error should be handled gracefully and not cause errors to be thrown out of the middleware.

This issue was also raised in #422 (direct quote link)

[codefromthecrypt]

ack. this would impact any middleware. There's a slight problem which is that we are writing bug guards and the bug guards could have bugs.

anyway we can try to figure a way to best effort some protection assuming express restify etc don't already have a middleware guard

[jcchavezs]

@russell-matt any chance you can scratch a failing test for this case?