ngMessage is a directive with the purpose to show and hide a particular message.
For ngMessage to operate, a parent ngMessages directive on a parent DOM element
must be situated since it determines which messages are visible based on the state
of the provided key/value map that ngMessages listens on.
More information about using ngMessage can be found in the
ngMessages module documentation.
<!-- using attribute directives -->
<ANY ng-messages="expression" role="alert">
  <ANY ng-message="stringValue">...</ANY>
  <ANY ng-message="stringValue1, stringValue2, ...">...</ANY>
</ANY>
<!-- or by using element directives -->
<ng-messages for="expression" role="alert">
  <ng-message when="stringValue">...</ng-message>
  <ng-message when="stringValue1, stringValue2, ...">...</ng-message>
</ng-messages>
  
  | Param | Type | Details | 
|---|---|---|
| ngMessage | when | expression | 
         a string value corresponding to the message key.  |