add initial implementation of fn call converter

openhands/core/exceptions.py

@@ -94,3 +94,21 @@ class CloudFlareBlockageError(Exception):
     """Exception raised when a request is blocked by CloudFlare."""
 
     pass
+
+
+class FunctionCallConversionError(Exception):
+    """Exception raised when FunctionCallingConverter failed to convert a non-function call message to a function call message.
+
+    This typically happens when there's a malformed message (e.g., missing <function=...> tags). But not due to LLM output.
+    """
+
+    pass
+
+
+class FunctionCallValidationError(Exception):
+    """Exception raised when FunctionCallingConverter failed to validate a function call message.
+
+    This typically happens when the LLM outputs unrecognized function call / parameter names / values.
+    """
+
+    pass