SUPP: More informative IntegrityError on duplicate columns

[oasbrink]

Closes #1359

[pep8speaks]

Hello @oasbrink! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2019-09-06 15:33:59 UTC

[codecov]

Codecov Report

Merging #1949 into master will decrease coverage by 1.65%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1949      +/-   ##
==========================================
- Coverage   87.67%   86.01%   -1.66%     
==========================================
  Files          93       93              
  Lines       16968    16971       +3     
  Branches     2144     2145       +1     
==========================================
- Hits        14876    14598     -278     
- Misses       1683     1966     +283     
+ Partials      409      407       -2

Impacted Files	Coverage Δ
ibis/expr/schema.py	90.1% <100%> (-0.8%)	⬇️
ibis/bigquery/client.py	41.1% <0%> (-53.39%)	⬇️
ibis/bigquery/compiler.py	59.92% <0%> (-37.5%)	⬇️
ibis/bigquery/udf/api.py	80.48% <0%> (-14.64%)	⬇️
ibis/impala/compiler.py	91.23% <0%> (-5.2%)	⬇️
ibis/pandas/client.py	85.54% <0%> (-3.47%)	⬇️
ibis/bigquery/api.py	63.33% <0%> (-3.34%)	⬇️
ibis/sql/compiler.py	94.26% <0%> (-0.39%)	⬇️
ibis/expr/api.py	92.33% <0%> (-0.37%)	⬇️