Ensure docs are complete and correct with respect to SEVERE vs ERROR

[dlebauer]

Written while reviewing of PecanProject#3589

@infotroph please advise if you prefer I target this PR to pecan/develop instead of your PR.

Summary

• Updates documentation to make sure SEVERE log level is consistently documented
• Fixes a bug in logger.getLevelNumber where ERROR and SEVERE were assigned the same value. I believe that this is why logger.severe does not stop workflows like it should.
• Clarifies that logger.setLevel should be used instead of logger.getLevel for temporary changes (since I realized during review of 3589 that setLevel returns the old level)

Question for discussion

What is the desired behavior for SEVERE, and what is the desired default?

My opinion: by default, logger.severe should stop execution and throw an error that can be caught in testing with testthat::expect_error.

Currently, the logger.setQuitOnSevere docs are unclear:

First, the default set at top of logger.R

.utils.logger$quit <- FALSE

But the logger.setQuitOnSevere docs indicate that default is TRUE, although I don't fully understand the desired behavior when run interactively, inside Rstudio, or when scripted. And what makes Rstudio special? Is that just a shorthand for 'interactive' - would the same thing happen when run in Rstudio as a background job?

##' set \code{\link{logger.setQuitOnSevere}(FALSE)} to avoid terminating
##' the session. This is set by default to TRUE if interactive or running
##' inside Rstudio.

[infotroph]

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