Improve error messages through proxy
Improve error messages through proxy

Previously, the proxy would only report the error code it received from
an InfluxDB instance. This also passes the error message returned by
InfluxDB through, so it appears in the Chronograf server logs as well as
in the response returned to the frontend by the proxy (making it visible
by inspecting the request in the network tab).

influxdb/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,li-ang/influxdb,influxdb/influxdb,nooproblem/influxdb,influxdb/influxdb,influxdata/influxdb,nooproblem/influxdb,influxdb/influxdb,influxdata/influxdb,influxdb/influxdb,influxdb/influxdb,li-ang/influxdb,mark-rushakoff/influxdb