Catch KeyError on missing sections and warn

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
@@ -101,6 +101,12 @@ def sort_imports(
                 )
             except FileSkipped:
                 skipped = True
+            except KeyError as error:
+                if error.args[0] in DEFAULT_CONFIG.sections:
+                    incorrectly_sorted = True
+                    warn(f"Cannot parse {file_name} because {error} missing in `sections` config")
+                else:
+                    raise
             return SortAttempt(incorrectly_sorted, skipped, True)
     except (OSError, ValueError) as error:
         warn(f"Unable to parse file {file_name} due to {error}")