Implement OB error class in taxonomy_units

[cwhanse]

Also, some edits to method logic for clarity

[cwhanse]

I'm not sure I follow. If unit_str isn't a valid unit_id, unit_name or id then get_unit will raise the error and terminate. I don't think it will ever pass None back as the unit. Are you saying that you'd prefer that get_unit pass back None and an error message, rather than raising the error?

[cwhanse]

An alternative logic for cli.py:

def list_unit_details(args):
    try:
        unit = taxonomy.units.get_unit(args.unit)
         print...
    except OBNotFoundError:
          print("Not found")

[joelebwf]

The alternative logic looks fine to me. Feel free to implement and merge.

I originally programmed the get_ statements to pas None for "Not Found". That is not necessarily best - just the way I did things. I think we should just keep the discussion going on how to return errors.

[cwhanse]

OK. The other calls to get_unit in cli.list_units_details are OK, since they are within a loop over a list of taxonomy units, rather than some other input.

[codecov-io]

Codecov Report

Merging #138 into master will increase coverage by 0.22%. The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
+ Coverage   89.49%   89.72%   +0.22%     
==========================================
  Files          25       25              
  Lines        2466     2462       -4     
==========================================
+ Hits         2207     2209       +2     
+ Misses        259      253       -6

Impacted Files	Coverage Δ
oblib/taxonomy_units.py	84.74% <50%> (+4.41%)	:arrow_up:
oblib/validator.py	83.33% <0%> (ø)	:arrow_up:

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