Add message to server tx query result in case of issues

[holtgrewe]

Is your feature request related to a problem? Please describe.
After #385 we return an empty page for missing genes. We should add some error/warning details message to the result.

Describe the solution you'd like
Add error/warning message to the result. This needs further specification and should include a computer-readable level (info/warning/error), computer-readable code, and a human-readable message.

Describe alternatives you've considered
N/A

Additional context

• Server /genes/txs should not return HTTP 500 on missing gene #385