Show Name of Undefined Control Sequence (macro)

[Strauman]

When LaTeX complains about undefined control sequence, this plugin does
not show the name of said macro, which is what this commit attempts to
fix.

[Strauman]

Simple LaTeX test case:

\documentclass{minimal}
\def\once{Once}
\begin{document}
  \once upon a time there was an \undefined control sequence.
\end{document}

Expected log output:

[Strauman]

I'm not highly confident as to what ideally should a legal macro name in
this setting.

The problem is that the escape character, \ by default, can be redefined, as well
as having anything being active letters (and thus treated as control sequences)

E.g. doing

\catcode`O=13\relax O

would make the letter O an unknown control sequence, and

\catcode`J=0\relax JUndefined

would make JUnedfined behave identically to \Undefined and this atom plugin
will report weird information.

You can't just look at the last space and go from there. If you consider

{}\some@undef@macro

Then the {} would be a part of the macro name reported by atom-latex.

However, I do argue that these cases are extraordinary circumstances,
and isn't really that necessary to implement.

[thomasjo]

I would really love it if we had a test covering this. The easiest way would probably be to use one of the log files in the fixtures folder (e.g. file.log) and augment it to contain an undefined control sequence error. Alternatively create a file which produces that error, and alter the locally generated log file by censoring local file paths, etc, like we've done for existing fixtures.

Having a test covering at least the "happy path" of this new logic ensures correct behavior, guards against regressions, and what not. Having tests that cover cases where a macro is not found would be amazing, but not strictly required.

[Strauman]

Having tests that cover cases where a macro is not found would be amazing, but not strictly required.

I'll see if I get the time.