Improve ruffus exception handling

ruffus swallows the return code if the process of handling an exception
we hit an error in ruffus' own code, which can happen.  So pick through
its error stack and find out if there's an interesting return code in
there.  Had to use eval() of all things.

Also suppress the stack trace for normal error conditions that don't
need one.

tests/resources/invalid.pdf

@@ -0,0 +1,3 @@
+%PDF-1.3
+This is not a valid PDF file
+%%EOF