Return helpful error when plotting model before running model

[apulsipher]

When plot(model) is called before run(model), plot throws an error and two warnings:

Error in if (res < 1000) return(1) : missing value where TRUE/FALSE needed
In addition: Warning messages:
1: In max(curves$counts) : no non-missing arguments to max; returning -Inf
2: In find_scale(max(curves$counts)) : NaNs produced

This doesn't tell the user that the error might be related to not running the model beforehand.

The error should be updated to give helpful info to the user and suggest running the model first. Also, the warnings may not be necessary.

When resolving this issue, make sure to update relevant unit tests to check for the specific error message pattern.