Add more error handling to the ECR backend

The error messages were copied from `botocore`.
New exceptions: RepositoryNotFoundException & ImageNotFoundException.

moto/ecr/exceptions.py

@@ -0,0 +1,22 @@
+from __future__ import unicode_literals
+from moto.core.exceptions import RESTError
+
+
+class RepositoryNotFoundException(RESTError):
+    code = 400
+
+    def __init__(self, repository_name, registry_id):
+        super(RepositoryNotFoundException, self).__init__(
+            error_type="RepositoryNotFoundException",
+            message="The repository with name '{0}' does not exist in the registry "
+                    "with id '{1}'".format(repository_name, registry_id))
+
+
+class ImageNotFoundException(RESTError):
+    code = 400
+
+    def __init__(self, image_id, repository_name, registry_id):
+        super(ImageNotFoundException, self).__init__(
+            error_type="ImageNotFoundException",
+            message="The image with imageId {0} does not exist within the repository with name '{1}' "
+                    "in the registry with id '{2}'".format(image_id, repository_name, registry_id))