bug#46295: syntax error in package definition leads to misleading error message

[Ricardo Wurmus]

In (gnu packages cran) I added a snippet but forgot to quote it
properly.  The snippet looked like this:

  (snippet
   (delete-file "inst/htmlwidgets/lib/jquery.min.js"))

Since this is evaluated right away I got an error as I tried to compile
my Guix source checkout:

   error: failed to load 'gnu/packages/cran.scm': No such file or directory

This error is highly misleading as it implies that
“gnu/packages/cran.scm” could not be found, while the actual error is
that “inst/htmlwidgets/lib/jquery.min.js” does not exist.  This could be
improved by catching 'system-error and reporting the file location
instead of just printing the correct error message without context.

-- 
Ricardo