Fix on_denied and on_missing documentation bugs

The default value for on_denied should be "error", while the default
value for on_missing is incorrectly listed as "error" in the
documentation.

plugins/lookup/aws_ssm.py

@@ -60,7 +60,7 @@
         - C(error) will raise a fatal error when the SSM parameter is missing.
         - C(skip) will silently ignore the missing SSM parameter.
         - C(warn) will skip over the missing SSM parameter but issue a warning.
-    default: error
+    default: skip
     type: string
     choices: ['error', 'skip', 'warn']
     version_added: 2.0.0
@@ -174,7 +174,7 @@ class LookupModule(LookupBase):
     def run(self, terms, variables=None, boto_profile=None, aws_profile=None,
             aws_secret_key=None, aws_access_key=None, aws_security_token=None, region=None,
             bypath=False, shortnames=False, recursive=False, decrypt=True, on_missing="skip",
-            on_denied="skip"):
+            on_denied="error"):
         '''
             :arg terms: a list of lookups to run.
                 e.g. ['parameter_name', 'parameter_name_too' ]