Replace usage of berrors.ServerInternal

[jsha]

Follow up to #6050. Any plain error returned via gRPC will get translated by WFE into a JSON Problem Document with "type": "ietf...serverInternal"), regardless of whether we wrap it in berrors.ServerInternal. We should eliminate use of berrors.ServerInternal since it's redundant.

[beautifulentropy]

I think, s/berrors.ServerInternal/berrors.InternalServer/