feat($controller): throw when requested controller is not registered

Previously, it would throw the ng:areq error, which is less
specific and just informs that the requested controller is not defined. 
Given how commonly controllers are used
in Angular, it makes sense to have a specific error.

The ng:areq error is still thrown when the registered controller
is not a function.

Closes angular#14980
PR (angular#15015)

docs/content/error/$controller/ctrlreg.ngdoc

@@ -0,0 +1,23 @@
+@ngdoc error
+@name $controller:ctrlreg
+@fullName A controller with this name is not registered.
+@description
+
+This error occurs when the {@link ng.$controller `$controller()`} service is called
+with a string that does not match any of the registered controllers. The controller service may have
+been invoked directly, or indirectly, for example through the {@link ng.ngController `ngController`} directive,
+or inside a {@link angular.Module#component component} / {@link angular.Module#directive directive} /
+{@link ngRoute.$routeProvider#when route} definition (when using a string for the controller property).
+Third-party modules can also instantiate controllers with the {@link ng.$controller `$controller()`} service.
+
+Causes for this error can be:
+
+1. Your reference to the controller has a typo. For example, in
+the {@link ng.ngController `ngController`} directive attribute, in a {@link angular.Module#component component}
+definition's controller property, or in the call to {@link ng.$controller `$controller()`}.
+2. You have not registered the controller (neither via {@link angular.Module#controller `Module.controller`}
+nor {@link ng.$controllerProvider#register `$controllerProvider.register()`}.
+3. You have a typo in the *registered* controller name.
+
+
+Please consult the {@link ng.$controller $controller} service api docs to learn more.

docs/content/error/ng/areq.ngdoc

@@ -5,4 +5,5 @@
 
 AngularJS often asserts that certain values will be present and truthy using a
 helper function. If the assertion fails, this error is thrown. To fix this problem,
-make sure that the value the assertion expects is defined and truthy.
+make sure that the value the assertion expects is defined and matches the type mentioned in the
+error.

src/ng/controller.js

@@ -122,6 +122,11 @@ function $ControllerProvider() {
             : getter(locals.$scope, constructor, true) ||
                 (globals ? getter($window, constructor, true) : undefined);
 
+        if (!expression) {
+          throw $controllerMinErr('ctrlreg',
+            'The controller with the name \'{0}\' is not registered.', constructor);
+        }
+
         assertArgFn(expression, constructor, true);
       }
 

test/ng/controllerSpec.js

@@ -161,6 +161,12 @@ describe('$controller', function() {
     }).toThrow();
   }));
 
+  it('should throw ctrlreg when the controller name does not match a registered controller', function() {
+    expect(function() {
+      $controller('IDoNotExist', {$scope: {}});
+    }).toThrowMinErr('$controller', 'ctrlreg', 'The controller with the name \'IDoNotExist\' is not registered.');
+  });
+
 
   describe('ctrl as syntax', function() {
 
@@ -227,7 +233,6 @@ describe('$controller', function() {
                        'Must match `__name__ as __id__` or `__name__`.');
     });
 
-
     it('should allow identifiers containing `$`', function() {
       var scope = {};
 

test/ng/directive/ngControllerSpec.js

@@ -150,4 +150,5 @@ describe('ngController', function() {
     $httpBackend.flush();
     expect(controllerScope.name).toBeUndefined();
   }));
+
 });