[AUCTeX] more helpful error messages when not using LaTeX

[jfbu]

Hi,

there was this thread

http://lists.gnu.org/archive/html/bug-auctex/2015-10/msg00000.html

on the bug tracker, I wish to add here a bit of additional
experience.

Consider this test file

\font\test=[lmmono10-regular]:

\test

I know this is no correct syntax.

\bye

and compile with C-cC-c the we get

ERROR: Font \test=[lmmono10-regular]: not loadable: Metric (TFM) file not found.

--- TeX said ---
<to be read again> 
                   \par 
l.2 
    
--- HELP ---
You requested a family/series/shape/size combination that is totally
unknown.  There are two cases in which this error can occur:
  1) You used the \size macro to select a size that is not available.
  2) If you did not do that, go to your local `wizard' and
     complain fiercely that the font selection tables are corrupted!

//end of error buffer

This is a pure LaTeX message which refers to the NFSS font system.

The command is 
Running `TeX' on `testmissingfont' with ``pdfetex  -file-line-error   
-interaction=nonstopmode "\input" testmissingfont.tex''

If I run this on the command line (removing -interaction=nonstopmode) I get:

$ pdfetex  -file-line-error   "\input" testmissingfont.tex
This is pdfTeX, Version 3.14159265-2.6-1.40.17 (TeX Live 2016) (preloaded 
format=pdfetex)
 restricted \write18 enabled.
entering extended mode
(./testmissingfont.texkpathsea:make_tex: Invalid fontname 
`[lmmono10-regular]:', contains '['

./testmissingfont.tex:2: Font \test=[lmmono10-regular]: not loadable: Metric (T
FM) file not found.
<to be read again> 
                   \par 
l.2 
    
? H
I wasn't able to read the size data for this font,
so I will ignore the font specification.
[Wizards can fix TFM files using TFtoPL/PLtoTF.]
You might try inserting a different font spec;
e.g., type `I\font<same font id>=<substitute font name>'.

? X
No pages of output.
Transcript written on testmissingfont.log.


which is the original error message.

I wonder if AUCTeX could consider separate error handling according
to engines. It is really a disturbance to see these LaTeX error
messages pop up.

Is the problem due to the fact that one may have to extract
from tex.web the original Knuth error messages ?

because the whole thing is run with -interaction=nonstopmode hence
AUCTeX must « own » its own database of help messages, those
the user gets on typing H in interactive mode ?

Best, Jean-François