Dashboard : common.bad_request isn't meaningful

[jansu-dev]

Feature Request

Is your feature request related to a problem? Please describe:

when customer uses dashboard to export slow.log or tidb.log. This error named common.bad_request isn't meaningful. I'm gonna list two scenarios I met to show why

1. Dump slow queries failed because of priv limit.

2. Slow query download from the TiDB dashboard was failing due to OOM at tidb server, customer had to increase tidb_mem_quota_query for the download worked. More info : https://pingcap.zendesk.com/agent/tickets/1910

Describe the feature you'd like:

I hope the error can give me a guidance or where it happens, so that I can run it correctly.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: