Steps to reproduce: create test.tex with

\documentclass{amsart}
\newtheorem{theorem}{Theorem}
\newtheorem{proposition}{Proposition}
\begin{document}
\begin{theorem}
  blah
\end{proposition}
\end{document}

and do C-c C-c followed by C-c `.  This jumps to the error, but no help is
available, even though LaTeX-error-description-list contains a relevant
entry.

The issue is that tex generates an error like the following:

LaTeX Error: \begin{theorem} on input line 5 ended by \end{proposition}.

On the other hand, the relevant regular expression in
LaTeX-error-description-list doesn't allow for "on input line X".  This
patch generalizes that regular expression so that it matches such output.

Thanks, best,

Paul