plugin: Report an error if a collector's interval cannot be determined.
plugin: Report an error if a collector's interval cannot be determined.

Else, we'll get weird behavior when, for example, using LoadPlugin instead of
LoadBackend in sysdbd.

tokkee/sysdb,sysdb/sysdb,tokkee/sysdb,sysdb/sysdb,sysdb/sysdb,tokkee/sysdb