Log Message:
latexmk: update to 456.

From v. 4.55 to 4.55a
  Add option that latexmk returns a non-zero status code when
    the final run of (pdf)latex gives warnings about undefined
    references or citations or about multiply defined references.
  Implement $warning_cmd to allow a visual cue for these situations in
    pvc mode (in addition to the current possibilities for
    $compiling_cmd, $success_cmd and $failure_cmd).

From v. 4.55a to 4.56
  Correct parsing of biber's log file.
  Implement $warning_cmd.
  Deal correctly with situation that files reported in the .fls file
    as INPUT or OUTPUT files no longer exist at end of run. These
    files aren't true source files or generated files from latexmk's
    point of view.  Such cases can arise from a bug in the TeX
    engine (as with lualatex in TeXLive 2016/2017).  They can also
    arise when temporary files are created, read and deleted during
    a run (as with the minted package).
  Update example latexmrc file for pythontex.
  Better error message when .bib file(s) not found.
  Better handling and diagnostics when names of expected and actual
    names of .log disagree.
  In particular, there is some automatic reconfiguration when .log
    file is not in expected place (e.g., because $aux_dir is different
    to $out_dir, but TeXLive is used, so -aux-directory option for TeX
    engines isn't supported).
  Fix problem of .bib files sometimes not being found by bibtex when
    latexmk runs under msys and -outputdirectory is specified.
  Add use of environment variable LATEXMKRCSYS to specify system rc
    file.