Give better error messages for failing to parse a spec

Call make_specs() explicitly and handle exceptions from it in order to
show the appropriate error message instead of letting argparse call it
implicitly. As mentioned in that module documentation:

> In general, the type keyword is a convenience that should only be used
> for simple conversions that can only raise one of the three supported
> exceptions. Anything with more interesting error-handling or resource
> management should be done downstream after the arguments are parsed.

And this is exactly what this commit does.

Closes #117.

abi3audit/_cli.py

@@ -178,7 +178,6 @@ def main() -> None:
     )
     parser.add_argument(
         "specs",
-        type=make_specs,
         metavar="SPEC",
         nargs="+",
         help="the files or other dependency specs to scan",
@@ -230,12 +229,20 @@ def main() -> None:
     if args.debug:
         logging.root.setLevel("DEBUG")
 
+    specs = []
+    for spec in args.specs:
+        try:
+            specs.extend(make_specs(spec))
+        except InvalidSpec as e:
+            console.log(f"[red]:thumbs_down: processing error: {e}")
+            sys.exit(1)
+
     logger.debug(f"parsed arguments: {args}")
 
     results = SpecResults()
     all_passed = True
     with status:
-        for spec in itertools.chain.from_iterable(args.specs):
+        for spec in specs:
             status.update(f"auditing {spec}")
             try:
                 extractor = spec._extractor()