Error reporting improvements

[edwintorok]

I like the error messages from Lambtex more than the LaTeX ones already, and the highlighting is definetely useful, but here are a few things to make them even better:

• lambcmd's stderr is completely silent, and I was surprised at first that I got the error messages in the output document. It would be useful to show some minimal output on stderr, for example: Failed to process '%s': %d errors encountered, see '%s' for details
• The error messages show just line numbers, however in some situations line1:column1 -- line2:column2 highlighted ranges would be more useful (got used to these kinds of messages from the OCaml compiler)
• When the wrapper block doesn't find the nested block it expects, or finds the wrong nested block it just prints syntax error. Please provide some hints in this case on what is wrong, for example:

\begin{bib}
\what{x}
\where{y}
\end{bib}

the bib block at line X requires \who, \what, and \where, but you didn't provide the \what command.

\begin{bib}
\picture{x}{y}
\who{z}
\what{x}
\where{y}
\end{bib}

the command \picture is not valid inside a bib block, valid commands are: \who, \what, \where

\subtitle{
\begin{foo}
\end{foo}
}

you cannot start a block command in an inline context, inline context started by \subtitle at line X.

  * runaway/missing end block detection could be improved for `source` blocks, for example instead of just reporting a syntax error at EOF here:

\begin{printout} \begin{source} abc \end{listing} \end{printout}

some more stuff EOF

a better error message would be:
`block 'source' is open at line X, but never closed.`

  * similar improvements should be made to detect missing `}` for inline commands, for example `\tt{`.

[darioteixeira]

I'm aware of that syntax errors are obtuse, though fixing them may not be easy with the current Menhir-based parser. For this and other reasons, I'm seriously considering rewriting the Lambtex parser using a different technology...