docs($compile): improve nonassign error message

- Improve error message to mention attribute the expression errored on

Fixes #13827

Closes #13828

src/ng/compile.js

@@ -2660,8 +2660,8 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
               // reset the change, or we will throw this exception on every $digest
               lastValue = destination[scopeName] = parentGet(scope);
               throw $compileMinErr('nonassign',
-                  "Expression '{0}' used with directive '{1}' is non-assignable!",
-                  attrs[attrName], directive.name);
+                  "Expression '{0}' in attribute '{1}' used with directive '{2}' is non-assignable!",
+                  attrs[attrName], attrName, directive.name);
             };
             lastValue = destination[scopeName] = parentGet(scope);
             var parentValueWatch = function parentValueWatch(parentValue) {

test/ng/compileSpec.js

@@ -3977,7 +3977,7 @@ describe('$compile', function() {
 
         componentScope.ref = 'ignore me';
         expect(function() { $rootScope.$apply(); }).
-            toThrowMinErr("$compile", "nonassign", "Expression ''hello ' + name' used with directive 'myComponent' is non-assignable!");
+            toThrowMinErr("$compile", "nonassign", "Expression ''hello ' + name' in attribute 'ref' used with directive 'myComponent' is non-assignable!");
         expect(componentScope.ref).toBe('hello world');
         // reset since the exception was rethrown which prevented phase clearing
         $rootScope.$$phase = null;
@@ -3994,7 +3994,7 @@ describe('$compile', function() {
 
         componentScope.ref = 'ignore me';
         expect(function() { $rootScope.$apply(); }).
-            toThrowMinErr("$compile", "nonassign", "Expression 'undefined' used with directive 'myComponent' is non-assignable!");
+            toThrowMinErr("$compile", "nonassign", "Expression 'undefined' in attribute 'ref' used with directive 'myComponent' is non-assignable!");
         expect(componentScope.ref).toBeUndefined();
 
         $rootScope.$$phase = null; // reset since the exception was rethrown which prevented phase clearing
@@ -4051,7 +4051,7 @@ describe('$compile', function() {
           componentScope.reference = {name: 'b'};
           expect(function() {
             $rootScope.$apply();
-          }).toThrowMinErr("$compile", "nonassign", "Expression '{name: name}' used with directive 'myComponent' is non-assignable!");
+          }).toThrowMinErr("$compile", "nonassign", "Expression '{name: name}' in attribute 'reference' used with directive 'myComponent' is non-assignable!");
 
         }));