[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.

lib/IRGen/GenMeta.cpp

@@ -5499,8 +5499,8 @@ SpecialProtocol irgen::getSpecialProtocolID(ProtocolDecl *P) {
   switch (*known) {
   case KnownProtocolKind::AnyObject:
     return SpecialProtocol::AnyObject;
-  case KnownProtocolKind::ErrorProtocol:
-    return SpecialProtocol::ErrorProtocol;
+  case KnownProtocolKind::Error:
+    return SpecialProtocol::Error;
     
   // The other known protocols aren't special at runtime.
   case KnownProtocolKind::Sequence: