fix(api/gateway): Handle err from tx properly to avoid panic

[renaynay]

Fixes a panic that can occur if txResp is nil and also returns actual error in the case of nil txResp.

Reported by @tuxcanfly

[codecov-commenter]

Codecov Report

Merging #2393 (7361749) into main (f32c903) will decrease coverage by 0.16%.
The diff coverage is 37.50%.

@@            Coverage Diff             @@
##             main    #2393      +/-   ##
==========================================
- Coverage   50.74%   50.58%   -0.16%     
==========================================
  Files         157      157              
  Lines        9925     9925              
==========================================
- Hits         5036     5021      -15     
- Misses       4445     4459      +14     
- Partials      444      445       +1     

Impacted Files	Coverage Δ
api/gateway/state.go	9.04% <37.50%> (-0.51%)	⬇️

... and 4 files with indirect coverage changes

[Wondertan]

Good bad request usage.
LGTM, Thank you!