bug#3930: DocView failure - bad error message, insufficient documentation

[David Reitter]

Trying to open a PDF file with Emacs/NS 23.1 branch (actually  
Aquamacs, which has no relevant patches compared to the 23.1 branch),  
I keep getting a "DocView: process pdf/ps->png changed status to trace/ 
BPT trap." message in *Messages*.

This happens with all PDFs, even small PDF files.

The buffer that is shown is not updated to reflect the error status.

What's worse is that the error message that is given is absolutely  
unhelpful.  It's unclear what "trace" or "BPT trap" means in this  
concept, or what the abbreviation "BPT" is supposed to mean.  This  
does not help me as a user in addressing the error condition.

I suspect that it needs some ominous conversion tool that is not  
installed or not installed in the right version.

I also looked up the doc string to "doc-view-mode", but it doesn't say  
anything about pre-requisites.

I finally found something in the source code, but looking  that up is  
not obvious and too much to ask of a user (who is not a dev).  Also,  
even with the given information, I can't tell which of the conversion  
tools it tried to call with what arguments, i.e. which one failed.

This needs a much better error message and ideally a bit more  
documentation.

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