Better error for external BigQuery tables.

[robertwb]

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[codecov]

Codecov Report

Merging #22178 (ae36588) into master (74a2a1b) will increase coverage by 0.14%.
The diff coverage is 37.50%.

@@            Coverage Diff             @@
##           master   #22178      +/-   ##
==========================================
+ Coverage   74.23%   74.38%   +0.14%     
==========================================
  Files         702      702              
  Lines       92873    93620     +747     
==========================================
+ Hits        68945    69637     +692     
- Misses      22661    22716      +55     
  Partials     1267     1267              

Flag	Coverage Δ
python	83.72% <37.50%> (+0.10%)	⬆️

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

Impacted Files	Coverage Δ
...ython/apache_beam/io/gcp/bigquery_read_internal.py	53.36% <0.00%> (-1.05%)	⬇️
sdks/python/apache_beam/io/gcp/bigquery.py	69.93% <75.00%> (-0.31%)	⬇️
sdks/python/apache_beam/io/source_test_utils.py	88.01% <0.00%> (-1.39%)	⬇️
sdks/python/apache_beam/runners/direct/executor.py	96.46% <0.00%> (-0.55%)	⬇️
sdks/python/apache_beam/typehints/schemas.py	94.32% <0.00%> (-0.34%)	⬇️
...hon/apache_beam/runners/worker/bundle_processor.py	93.42% <0.00%> (-0.25%)	⬇️
...dks/python/apache_beam/options/pipeline_options.py	94.76% <0.00%> (-0.21%)	⬇️
...apache_beam/runners/dataflow/internal/apiclient.py	77.34% <0.00%> (-0.05%)	⬇️
sdks/python/apache_beam/io/hadoopfilesystem.py	97.28% <0.00%> (ø)
... and 10 more

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[robertwb]

retest this please

[robertwb]

Run Python PreCommit

[pabloem]

lgtm thanks Robert