Failure in html output - Reference to nonexistent nodes

[Christopher Dimech]

Some time ago I discussed the problem that calling a reference
to a nonexistent node when generating .info files.

When outputting html, reference to nonexistent node also gives a
problem.  

@ref reference to nonexistent node `igc03--Sicl--CircAmRg'

Problem is aggravated because the failure stops the 
generation of any files.   
 
When using texi2pdf, one gets in the output pdf document

<undefined> [igc03–Sicl–CircAmRg], page <undefined>,

This is much better than not generating any output.

I have read some innovative ideas from Language Designer Luca
Saiu, to make programs run even with errors.  These ideas will
certainly be interesting, because as with the pdf output, the
difficulties are transplanted in the output.  Then one can
eventually go and fix them.

However, the problem with undefined ref will not stop the author
from continuing with his work until he resolves undefined ref.

I mention this also for the generation of .info files.