X.U.EZConfig: Force readKeySequence to consume the whole input

When users specify non-existent keys, it seems most intuitive to just
abort the parse and not try to take the "longest" input that still
works.  For example, given the "key" `M-10` we should signal a parse
error (by returning `Nothing`) instead of parsing `M-1` and ignoring
the rest of the input.  The old EZConfig parser accounted for this but
when the module was rewritten to use X.U.Parser in [1], this was
forgotten about.

Fixes: https://github.com/xmonad/xmonad/issues/361

[1]: 8abeb81fd0

tests/EZConfig.hs

@@ -25,6 +25,11 @@ spec = do
                          , mod2Mask, mod3Mask, mod4Mask, mod5Mask
                          ]]
 
+  -- Checking for regressions
+  describe "readKeySequence" $
+    it "Fails on the non-existent key M-10" $
+      readKeySequence def "M-10" `shouldBe` Nothing
+
 regularKeys :: ([String], [KeySym])
 regularKeys = unzip . map (first (: ""))
             $ zip ['!'    .. '~'   ] [xK_exclam       .. xK_asciitilde]