pip: propagate error when downloading packages

If pip fails to download, the generated sources file is empty and,
arguably, useless. Instead of silently generating an empty file, the
error
is propagated now.

The root cause appears to be a bug in pip.

#265

pip/flatpak-pip-generator

@@ -53,6 +53,8 @@ parser.add_argument('--runtime',
                     help='Specify a flatpak to run pip inside of a sandbox, ensures python version compatibility')
 parser.add_argument('--yaml', action='store_true',
                     help='Use YAML as output format instead of JSON')
+parser.add_argument('--ignore-errors', action='store_true',
+                    help='Ignore errors when downloading packages')
 opts = parser.parse_args()
 
 if opts.yaml:
@@ -251,6 +253,9 @@ with tempfile.TemporaryDirectory(prefix=tempdir_prefix) as tempdir:
     except subprocess.CalledProcessError:
         print('Failed to download')
         print('Please fix the module manually in the generated file')
+        if not opts.ignore_errors:
+            print('Ignore the error by passing --no-raise-on-error')
+            raise
 
     if not opts.requirements_file:
         try: