Improve diagnostic for Error compositions not being representable in Objective-C.

Thanks, Jordan!

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -2751,7 +2751,8 @@ NOTE(not_objc_empty_protocol_composition,none,
 NOTE(not_objc_protocol,none,
      "protocol %0 is not '@objc'", (Type))
 NOTE(not_objc_error_protocol_composition,none,
-     "protocol composition involving 'Error' is not '@objc'", ())
+     "protocol composition involving 'Error' cannot be represented "
+     "in Objective-C", ())
 NOTE(not_objc_empty_tuple,none,
       "empty tuple type cannot be represented in Objective-C", ())
 NOTE(not_objc_tuple,none,

test/attr/attr_objc.swift

@@ -1943,7 +1943,7 @@ class ClassThrows1 {
 
   @objc func fooWithErrorProtocolComposition1(x: Error & Protocol_ObjC1) { }
   // expected-error@-1{{method cannot be marked @objc because the type of the parameter cannot be represented in Objective-C}}
-  // expected-note@-2{{protocol composition involving 'Error' is not '@objc'}}
+  // expected-note@-2{{protocol composition involving 'Error' cannot be represented in Objective-C}}
 
   // CHECK: {{^}} func fooWithErrorProtocolComposition2(x: Error & Protocol_ObjC1)
   func fooWithErrorProtocolComposition2(x: Error & Protocol_ObjC1) { }