Issue warning if nothing is observed at an observation location

[jdhughes-usgs]

Is your feature request related to a problem? Please describe.
A no data value (3e30) is written if a specific boundary condition (location or boundname) does not exist in a stress period. Based on Issue #1939

Describe the solution you'd like
Write a warning message if nothing is observed at an observation location.

Describe alternatives you've considered
None.

Additional context
None.