Replacing experimental errorsource.DownstreamError with backend.DownstreamError logs a different errorSource

[idastambuk]

What happened:
Originally reported in grafana/grafana-plugin-sdk-go#1087 (comment)
We have a DownstreamError function in both the experimental package and backend package. However, if I replace the experimental one with the backend one, the error doesn't have the errorSource marked as downstream anymore (but the error string is prepended with downstream error).

Looks like we overwrite the source: https://raintank-corp.slack.com/archives/C068FL57F0V/p1726590417646519?thread_ts=1726590100.558479&cid=C068FL57F0V
What you expected to happen:

for the errorSource to be "downstream"

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• SDK version:
• Grafana version:
• Go version: