[Go SDK] minor error handling fixes

[lostluck]

Fix SDF error output, and fix coder printout in exec.DataSink.

Found in developing estimated watermark handling for Prism (See #24789).

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[codecov]

Codecov Report

Merging #25555 (8efbc1d) into master (6e08092) will decrease coverage by 0.16%.
The diff coverage is 61.25%.

@@            Coverage Diff             @@
##           master   #25555      +/-   ##
==========================================
- Coverage   72.77%   72.61%   -0.16%     
==========================================
  Files         749      758       +9     
  Lines       99434   100566    +1132     
==========================================
+ Hits        72362    73030     +668     
- Misses      25709    26127     +418     
- Partials     1363     1409      +46     

Flag	Coverage Δ
go	52.11% <61.25%> (+0.25%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/go/pkg/beam/core/runtime/exec/datasink.go	0.00% <0.00%> (ø)
...o/pkg/beam/runners/prism/internal/config/config.go	94.73% <ø> (ø)
...o/pkg/beam/core/runtime/exec/sdf_invokers_arity.go	31.91% <31.91%> (ø)
sdks/go/pkg/beam/core/runtime/exec/sdf.go	66.12% <49.38%> (-4.89%)	⬇️
...beam/runners/prism/internal/jobservices/metrics.go	70.83% <70.83%> (ø)
sdks/go/pkg/beam/core/runtime/exec/sdf_invokers.go	76.30% <86.53%> (+6.12%)	⬆️
sdks/go/pkg/beam/core/graph/fn.go	85.11% <95.23%> (+0.43%)	⬆️
sdks/go/pkg/beam/core/runtime/exec/datasource.go	68.93% <100.00%> (ø)
sdks/go/pkg/beam/io/mongodbio/read.go	9.03% <0.00%> (-21.94%)	⬇️
sdks/go/pkg/beam/io/mongodbio/coder.go	83.33% <0.00%> (-16.67%)	⬇️
... and 18 more

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