Custom checks: allow to specify error level (warning or error)

[kiryph]

my.bib:

@article{Koch:1984,
  author  = {Koch, E.},
  doi     = {10.1107/S0108767384001227},
  issn    = {0108-7673},
  journal = {Acta Crystallographica Section A Foundations of Crystallography},
  month   = {sep},
  number  = {5},
  pages   = {593--600},
  title   = {The implications of normalizers on group–subgroup relations between space groups},
  volume  = {40},
  year    = {1984}
}
@Article{Dove:1997,
  author  = {Dove, Martin T.},
  title   = {Theory of displacive phase transitions in minerals},
  doi     = {10.2138/am-1997-3-401},
  issn    = {0003-004X},
  journal = {American Mineralogist},
  month   = {apr},
  number  = {3-4},
  pages   = {213--244},
  volume  = {82},
  year    = {1997}
}
@article{Koch:1984,
  author  = {Koch, E.},
  doi     = {10.1107/S0108767384001227},
  issn    = {0108-7673},
  journal = {Acta Crystallographica Section A Foundations of Crystallography},
  month   = {sep},
  number  = {5},
  pages   = {593--600},
  title   = {The implications of normalizers on group–subgroup relations between space groups},
  volume  = {40},
  year    = {1984}
}

% Semantic checks
check.rule { month "^[\"{]1?[0-9][\"}]$" }
check.rule { month "" "\@ \$: Month has to be a suitable number\n" }

❯ bibtool -r .bibtoolrsc my.bib > /dev/null
*** BibTool: Article koch:1984: Month has to be a suitable number
*** BibTool: Article dove:1997: Month has to be a suitable number
*** BibTool: Article koch:1984: Month has to be a suitable number

I would like to specify that this is a warning not an error similar to

*** BibTool ERROR:  (line 3 in ./z.bib): Unexpected character encountered
*** BibTool WARNING:  (line 5 in ./z.bib): 17 non-space characters ignored.

The output should be like

❯ bibtool -r .bibtoolrsc my.bib > /dev/null
*** BibTool WARNING: Article koch:1984: Month has to be a suitable number
*** BibTool WARNING: Article dove:1997: Month has to be a suitable number
*** BibTool WARNING: Article koch:1984: Month has to be a suitable number

This is useful when bibtool is run as a linter and the messages are categorized into errors and warnings.

[ge-ne]

Resources check.warning.rule and check.error.rule introduced for this.