PParser.hs: filename: Explain failed parse with |fail|

This still provides misleading output from the parser about what it
was expecting, but does explain what the actual problem was:

    *Pkg.PParser> execout <$> runparser pPkg defaultPkg "foo.ipkg"
                  "package bar\n\nexecutable = \"invalid/filename\""
    Failure foo.ipkg:3:32: error: a filename
        must be non-empty and have no
        directory
        component, expected: space
    executable = "invalid/filename"<EOF>
                                   ^

src/Pkg/PParser.hs

@@ -79,8 +79,9 @@ filename = (do
         -- Through at least version 0.9.19.1, IPKG executable values were
         -- possibly namespaced identifiers, like foo.bar.baz.
         show <$> fst <$> iName []
-    guard $ isValidFilename filename
-    return filename)
+    if isValidFilename filename
+      then return filename
+      else fail "a filename must be non-empty and have no directory component")
     <?> "filename"
     where
         isValidFilename :: FilePath -> Bool