sqlutil.FrameFromRows: multiple results sets with different columns fail

[gabor]

try to run this query in postgres (may work in other databases too):

select 41; select 42 as A, 43 as B;

and call sqlutil.FrameFromRows on the result.

it fails with the error message: sql: expected 1 destination arguments in Scan, not 2

the PR #735 added support for handling multiple result sets, but it only works fine if every result-set has the same columns.

i'm not sure what the best solution would be:

• return multiple frames in this case (needs change to the API, currently it returns a single frame)
• behave as currently, but detect this problem and return a better error message
• something else