add internal error html page (forgotten from previous commits)
add internal error html page (forgotten from previous commits)

jbfavre/exaproxy,PrFalken/exaproxy,PrFalken/exaproxy,jbfavre/exaproxy,david-farrar/exaproxy,david-farrar/exaproxy,david-farrar/exaproxy,PrFalken/exaproxy