Handle malformed inputs given via the CLI more gracefully

Instead of crashing and exposing the traceback of pyasn1
internals, show an error message. This can help differentiate
an error inside pkilint from an error inside the input data.

pkilint/bin/lint_pkix_cert.py

@@ -44,7 +44,11 @@ def main(cli_args=None) -> int:
 
         return 0
     else:
-        cert = loader.load_certificate(args.file, args.file.name)
+        try:
+            cert = loader.load_certificate(args.file, args.file.name)
+        except ValueError as e:
+            print(f'Failed to load certificate: {e}')
+            return 1
 
         results = doc_validator.validate(cert.root)