Log null statsInfo at 'debug' instead of 'error'

[will-moore]

Since it's no longer an error to have no stats info (see #3580) we should log this merely at 'debug' level instead of 'error'.

To test:

• Import image with no stats
• Open in web - observe Web log for lack of errors.

--no-rebase.

[sbesson]

Setting omero.web.debug to True, I can confirm the StatsInfo message is displayed at DEBUG level instead of ERROR level with this PR included.
With regard to the impact of this change, I would consider this logging message important enough to promote it to the INFO level instead. With the exposition of --skip parameters, users may run into more of these issues and we may want to have this information logged by default in production instances. /cc @jburel

[jburel]

probably better to have it at INFO level as @sbesson suggested. not really a DEBUG

[sbesson]

2015-03-23 08:32:24,506  INFO [                           omero.gateway] (proc.30074) getWindowMin:6629 getStatsInfo() is null. See #9695
2015-03-23 08:32:24,507  INFO [                           omero.gateway] (proc.30074) getWindowMax:6650 getStatsInfo() is null. See #9695

Thanks