add missing exception

Author: KelvinChung2000

FABulous/custom_exception.py

@@ -0,0 +1,11 @@
+class CommandError(Exception):
+    """Exception raised for errors in the command execution."""
+    pass
+
+class EnvironmentNotSet(Exception):
+    """Exception raised when the environment is not set."""
+    pass
+
+class FileTypeError(Exception):
+    """Exception raised for unsupported file types."""
+    pass