Handle LaTeX's `\@missingfileerror`

[ktraunmueller]

Catch \@missingfileerror invocations and integrate with Compositor's error & warning reporting

\gdef\@missingfileerror#1#2{%
     \typeout{^^J! LaTeX Error: File `#1.#2' not found.^^J^^J%
      Type X to quit or <RETURN> to proceed,^^J%
      or enter new name. (Default extension: #2)^^J}%
      ...