fix unhandled error when output dir is not specified during collect

Pegleg is tossing an error that's difficult to decipher when an output
directory is not specified during "collect". Currently "save-location"
param is optional and can be missed during command execution. This PS
handles missing or invalid output directory exception.

Change-Id: I6f41e4951be29d8ecdce73f6090f0bcc9e7ee2d9

src/bin/pegleg/pegleg/cli.py

@@ -69,7 +69,6 @@ def site(primary_repo, aux_repo):
     'save_location',
     type=click.Path(
         file_okay=False, dir_okay=True, writable=True, resolve_path=True),
-    default=sys.stdout,
     help='Where to output')
 @click.argument('site_name')
 def collect(*, save_location, site_name):

src/bin/pegleg/pegleg/engine/site.py

@@ -29,6 +29,10 @@
 def collect(site_name, save_location):
     try:
         save_files = dict()
+        if save_location is None:
+            raise ValueError('Missing param: save-location')
+        elif not os.path.exists(save_location):
+            raise FileNotFoundError('Invalid save-location path')
         for (repo_base,
              filename) in util.definition.site_files_by_repo(site_name):
             repo_name = os.path.normpath(repo_base).split(os.sep)[-1]