utils: do not print tracebacks when extensions fail to load
utils: do not print tracebacks when extensions fail to load

As hotdoc now ships extensions that are expected to fail loading
under certain circumstances, it makes no sense to print the traceback
to stdout anymore. Log to stderr when debug is enabled only.

thiblahute/hotdoc,thiblahute/hotdoc,thiblahute/hotdoc,hotdoc/hotdoc,hotdoc/hotdoc,hotdoc/hotdoc,thiblahute/hotdoc,hotdoc/hotdoc,hotdoc/hotdoc,thiblahute/hotdoc,hotdoc/hotdoc,thiblahute/hotdoc