[FIX] owsql: Fixes for Custom SQL option

[lanzagar]

Issue

1. Custom SQL had a case-sensitive check for SELECT (lower case did not work correctly)
2. Only psycopg2.ProgrammingError was caught, but after refactoring the BackendError can be thrown as well

Description of changes

1. case-insensitive check
2. catch both errors

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #2456 into master will decrease coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2456      +/-   ##
==========================================
- Coverage   74.33%   74.32%   -0.01%     
==========================================
  Files         321      321              
  Lines       55988    55988              
==========================================
- Hits        41616    41614       -2     
- Misses      14372    14374       +2