Avoid a TypeError in flatten.

pospell.py

@@ -301,7 +301,7 @@ def parse_args():
         version="%(prog)s " + __version__ + " using hunspell: " + HUNSPELL_VERSION,
     )
     parser.add_argument("--debug", action="store_true")
-    parser.add_argument("-p", "--personal-dict", type=str)
+    parser.add_argument("-p", "--personal-dict", type=Path)
     parser.add_argument(
         "--modified", "-m", action="store_true", help="Use git to find modified files."
     )
@@ -313,6 +313,9 @@ def parse_args():
         help="Number of files to check in paralel, defaults to all available CPUs",
     )
     args = parser.parse_args()
+    if args.personal_dict is not None and not args.personal_dict.exists():
+        print(f"Error: dictionary {str(args.personal_dict)!r} not found.")
+        sys.exit(1)
     if args.drop_capitalized and args.no_drop_capitalized:
         print("Error: don't provide both --drop-capitalized AND --no-drop-capitalized.")
         parser.print_help()
@@ -350,7 +353,7 @@ def run_hunspell(language, personal_dict, input_lines):
             input=quote_for_hunspell(text for _, _, text in input_lines),
         )
     except subprocess.CalledProcessError:
-        return -1
+        return []
     return parse_hunspell_output(input_lines, output.splitlines())