fix(spend-logs): preserve error_message on ProxyException failures

[songkuan-zheng]

Summary

StandardLoggingPayloadSetup.get_error_information stored an empty
error_message field in spend_logs.metadata.error_information for
every ProxyException, even though both the HTTP response body and
the captured traceback contained the full human-readable text.

Result: dashboard "LLM Failure" rows became untriageable — operators
had to manually unpack metadata.error_information.traceback JSON
via psql to distinguish a 401 from a 429 or a budget overrun.

Root cause

ProxyException (litellm/proxy/_types.py:3453):

class ProxyException(Exception):
    def __init__(self, message, type, param, code=None, ...):
        self.message = str(message)        # sets attribute
        # NOTE: does NOT call super().__init__(message)
        # NOTE: does NOT define __str__

Therefore str(ProxyException(...)) returns the empty string, and
get_error_information was calling str(original_exception) to
populate error_message. Every auth/budget/quota rejection landed
with error_message="".

Fix

litellm/litellm_core_utils/litellm_logging.py:5138-5148 — prefer the
.message attribute, fall back to str(exc) for non-litellm exception
types:

message_attr = getattr(original_exception, "message", None)
if message_attr:
    error_message = str(message_attr)
else:
    error_message = str(original_exception) if original_exception else ""

.message is set uniformly by ProxyException and every
litellm.exceptions.* class (see litellm/exceptions.py), so the
change is backward-compatible. Plain Exception / ValueError /
third-party exceptions still resolve via the str() fallback.

What this is NOT

• Not a request-correctness bug — the 401/429/etc. responses to clients
  were always correct
• Not a billing bug — the failures never reached the cost calculator
• Purely an observability regression — only spend_logs was affected

Test plan

• New unit test: test_get_error_information_prefers_message_attribute_over_str
• New unit test: test_get_error_information_falls_back_to_str_when_no_message_attr
• Existing test test_get_error_information_error_code_priority still passes
• Existing test test_get_error_information still passes
• End-to-end verified against the proxy: bad-key 401 now stores the
  full "Authentication Error, Invalid proxy server token passed..."
  message in spend_logs.metadata.error_information.error_message
• Black 24.10.0 formatting clean

Out of scope

• Direct fix at ProxyException (adding super().__init__(message))
  was considered but rejected: that class is marked DO NOT MODIFY
  for OpenAI-compatibility mapping reasons; the .message-preference
  fix at the logging layer is local and risk-bounded.