Improve internal functions to check query and columns

[cvvergara]

Todo:

To (internal SQL) Throw PostgreSQL errmsg, hint and error code

• Internal functions
  • _pgr_checkquery
  • _pgr_checkcolumn
• re-throw from
  • pgr_extractVertices
  • pgr_findCloseEdges
  • pgr_degree
• Add tests for missing column and relation not found
• (pgtap) adjusting tests because of changes on internal SQL
• Document the changes