[SE-0112] Rename ErrorProtocol to Error.

This is bullet (5) of the proposed solution in SE-0112, and the last
major piece to be implemented.

unittests/SourceKit/Support/FuzzyStringMatcherTest.cpp

@@ -315,4 +315,4 @@ TEST(FuzzyStringMatcher, ShortUnconnectedMatches) {
   FuzzyStringMatcher m("abcd");
   EXPECT_GT(m.scoreCandidate("xaxbxcdxxxxxx"), m.scoreCandidate("xaxbxcxd"));
   EXPECT_GT(m.scoreCandidate("xaxbxc_d"), m.scoreCandidate("xaxbxcxd"));
-}
+}