fix(error handler): re-throw http-proxy missing target error

[chimurai]

Background:
With router usage and optional target (#512); Required target for http-proxy can sometimes be empty.

This PR will re-throw the error emitted from http-proxy.

req and res object are empty, so there is no graceful way to handle this error...

In these cases it recommended to set target as fallback.

[coveralls]

Coverage decreased (-0.08%) to 87.302% when pulling 1eb6a85 on fix-error-missing-target into 40e8763 on master.