Refine exit codes

- 2 for file not found
- 3 for invalid arguments
- 4 for missing dependencies
- 123 for unknown failures

src/darker/__main__.py

@@ -549,9 +549,8 @@ def main(  # pylint: disable=too-many-locals,too-many-branches,too-many-statemen
             rev2_repr = (
                 "the working tree" if revrange.rev2 == WORKTREE else revrange.rev2
             )
-            raise ArgumentError(
-                Action(["PATH"], "path"),
-                f"Error: Path(s) {missing_reprs} do not exist in {rev2_repr}",
+            raise FileNotFoundError(
+                f"Path(s) {missing_reprs} do not exist in {rev2_repr}"
             )
 
     # These paths are relative to `common_root`:
@@ -631,10 +630,18 @@ def main_with_error_handling() -> int:
     """Entry point for console script"""
     try:
         return main()
-    except (ArgumentError, DependencyError) as exc_info:
-        if logger.root.level < logging.WARNING:
-            raise
-        sys.exit(str(exc_info))
+    except FileNotFoundError as exc_info:
+        logger.error("%s (2)", exc_info)
+        return 2
+    except ArgumentError as exc_info:
+        logger.error("%s (3)", exc_info)
+        return 3
+    except DependencyError as exc_info:
+        logger.error("%s (4)", exc_info)
+        return 4
+    except Exception as exc_info:  # pylint: disable=broad-exception-caught
+        logger.error("%s (123)", exc_info)
+        return 123
 
 
 if __name__ == "__main__":