add error docs

Author: Narretz

docs/content/error/ngRef/noctrl.ngdoc

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+@ngdoc error
+@name ngRef:noctrl
+@fullName A controller for the value of `ngRefRead` could not be found on the element.
+@description
+
+This error occurs when the {@link ng.ngRef ngRef directive} specifies
+a value in `ngRefRead` that cannot be resolved to a directive / component controller.
+
+Causes for this error can be:
+
+1. Your `ngRefRead` value has a typo.
+2. You have a typo in the *registered* directive / component name.
+3. The directive / component does not have a controller.
+
+Note that `ngRefRead` takes the name of the component / directive, not the name of controller, and
+also not the combination of directive and 'Controller'. For example, for a directive called 'myDirective',
+the correct declaration is `<div ng-ref="$ctrl.ref" ng-ref-read="myDirective">`.

docs/content/error/ngRef/nonassign.ngdoc

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+@ngdoc error
+@name ngRef:nonassign
+@fullName Non-Assignable Expression
+@description
+
+This error occurs when ngRef defines an expression that is not-assignable.
+
+In order for ngRef to work, it must be possible to write the reference into the path defined with the expression.
+
+For example, the following expressions are non-assignable:
+
+```
+<my-directive ng-ref="{}"></my-directive>
+
+<my-directive ng-ref="myFn()"></my-directive>
+
+<!-- missing attribute value is also invalid -->
+<my-directive ng-ref></my-directive>
+
+```
+
+To resolve this error, use a path expression that are assignable:
+
+```
+<my-directive ng-ref="$ctrl.reference"></my-directive>
+
+```