Improve validation error message

[damccorm]

Right now, it isn't clear from this error which DoFn is causing this error/where it is. This will dump the whole DoFn's name and file path.

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[codecov]

Codecov Report

Merging #17719 (fab8c5d) into master (da4fcad) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #17719   +/-   ##
=======================================
  Coverage   73.99%   73.99%           
=======================================
  Files         695      695           
  Lines       91798    91798           
=======================================
  Hits        67926    67926           
  Misses      22624    22624           
  Partials     1248     1248           

Flag	Coverage Δ
go	50.41% <100.00%> (ø)

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Impacted Files	Coverage Δ
sdks/go/pkg/beam/core/graph/fn.go	76.82% <100.00%> (ø)

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[riteshghorse]

R: @lostluck

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Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[lostluck]

LGTM Thanks!