Gracefully fail on missing default sections

isort/main.py

@@ -14,7 +14,7 @@
 from .format import create_terminal_printer
 from .logo import ASCII_ART
 from .profiles import profiles
-from .settings import VALID_PY_TARGETS, Config, WrapModes
+from .settings import DEFAULT_CONFIG, VALID_PY_TARGETS, Config, WrapModes
 
 try:
     from .setuptools_commands import ISortCommand  # noqa: F401
@@ -109,6 +109,18 @@ def sort_imports(
         if config.verbose:
             warn(f"Encoding not supported for {file_name}")
         return SortAttempt(incorrectly_sorted, skipped, False)
+    except KeyError as error:
+        if error.args[0] not in DEFAULT_CONFIG.sections:
+            _print_hard_fail(config, offending_file=file_name)
+            raise
+        msg = (
+            f"Found {error} imports while parsing, but {error} was not included "
+            "in the `sections` setting of your config. Please add it before continuing\n"
+            "See https://pycqa.github.io/isort/#custom-sections-and-ordering "
+            "for more info."
+        )
+        _print_hard_fail(config, message=msg)
+        sys.exit(os.EX_CONFIG)
     except Exception:
         _print_hard_fail(config, offending_file=file_name)
         raise