ceph-mgr should log an error if it can't find any modules to load
I had a downstream SUSE build of ceph-14.1.0, which was showing "7 mgr modules have failed" in `ceph status`, and `ceph mgr dump` showed "available_modules":  (i.e. an empty list). The 7 failed modules were of course the always-on modules. I eventually figured out that mgr_module_path was somehow set to /usr/local/share/ceph/mgr in that build, but the modules were actually correctly installed in /usr/share/ceph/mgr, so mgr couldn't find them. We should log the path we're loading modules from, and log an error if none are found, so that if mgr_module_path is ever set incorrectly in future, the problem will be obvious.