[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.

test/SILGen/coverage_exceptions.swift

@@ -1,6 +1,6 @@
 // RUN: %target-swift-frontend -Xllvm -sil-full-demangle -profile-generate -profile-coverage-mapping -emit-sorted-sil -emit-sil -module-name coverage_catch %s | FileCheck %s
 
-enum SomeErr : ErrorProtocol {
+enum SomeErr : Error {
   case Err1
   case Err2
 }