Auto merge of #16752 - jdm:css-parse-error, r=SimonSapin

Report more informative CSS errors

This requires https://github.com/servo/rust-cssparser/pull/143 for the final commit. There's no better way to split that work up, unfortunately, and it's extremely easy to bitrot. I would appreciate if we could expedite reviewing this work.

This is the work necessary to enable https://bugzilla.mozilla.org/show_bug.cgi?id=1352669. It makes sense to merge it separately because it's so much effort to keep it up to date with the ongoing Stylo work.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes

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components/style/properties/longhand/ui.mako.rs

@@ -76,11 +76,12 @@ ${helpers.single_keyword("-moz-window-shadow", "none default menu tooltip sheet"
 
     impl ComputedValueAsSpecified for SpecifiedValue {}
 
-    pub fn parse(_context: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue, ()> {
+    pub fn parse<'i, 't>(_context: &ParserContext, input: &mut Parser<'i, 't>)
+                         -> Result<SpecifiedValue, ParseError<'i>> {
         match try!(input.expect_integer()) {
             0 => Ok(computed_value::T(false)),
             1 => Ok(computed_value::T(true)),
-            _ => Err(()),
+            _ => Err(StyleParseError::UnspecifiedError.into()),
         }
     }