fix: error code on query failure

[alanshaw]

Any error returned from the query engine is a server error - we've already validated the client sent parameters, it's no longer possible for the request to be considered bad.

I also switched magic constant 400 out for http.StatusBadRequest for readability.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/server/server.go	0.00%	3 Missing ⚠️

Files with missing lines	Coverage Δ
pkg/server/server.go	17.82% <0.00%> (ø)

... and 1 file with indirect coverage changes