[FR] Recognize L3 \msg_line_context: on warning parsing

[Gustavo Barros]

Hi All,

The LaTeX3 `l3msg' package defines the helper macro `\msg_line_context:' 
for use in messages, which is documented as:

"Prints the current line number when a message is given, and thus 
suitable for giving context to messages. The number itself is proceeded 
by the text 'on line'."

However, if one uses `\msg_line_context:' in their messages, AUCTeX 
fails to recognize the line of the message, and some convenience is lost 
because of that.

As far as I can tell, this is because this message differs from the 
traditional "on input line ...", and `TeX-warning' at `tex-buf.el' sets 
the let-bound `line-string' to `"on input line \\([0-9]*\\)\\."'.

WDYT, could `TeX-warning' be adjusted to accommodate 
`\msg_line_context:' too?

Best regards,
Gustavo.