Allow configuring multiple error procs

This updates Sanford hosts to allow configuring multiple error
procs. Sanford error handling will call all of them, but will
only use the last configured proc's response (if it generated
one). This should dictate that the most "specific" error proc's
response will be used. This is to allow flexibility of
configuring a host using mixins or other methods.
Merge pull request #66 from redding/jcr-allow-multiple-error-procs

Allow configuring multiple error procs