improve logging
improve logging

Previously, we just raised Exceptions for all errors, which resulted
in 500 internal server errors being returned to callers.
Now graphite returns errors based on the problem. eg, if metrictank returns
a 400 error, graphite will also return a 400 error.  If metrictank is
unreachable, graphite will return a 503 error.

raintank/graphite-raintank,raintank/graphite-raintank,raintank/graphite-raintank