Fail early and descriptively if user doesn't ask for coverage on any resources

[isc-tleavitt]

For background, see https://community.intersystems.com/post/errors-when-using-testcoverage-somebody-else-using-monitor#comment-178651

[isc-tleavitt]

Actually - this might also cover #10 if we put the check once we have the list of routines to monitor. (It's also an error if we say to monitor things but there's no .INT there.)

[isc-tleavitt]

#12 fixes this.