Spec fix: simple_query can return multiple errors

simple_query/{2,3,4} can return both an error and result.

Examples:

  pgsql_connection:simple_query("CREATE TABLE a (n integer); CREATE TABLE a (n integer);", C).
  > [
  >   {error,{pgsql_error,[{severity,<<"ERROR">>},
  >                        {code,<<"42P07">>},
  >                        {message,<<"relation \"a\" already exists">>},
  >                        {file,<<"heap.c">>},
  >                        {line,<<"1047">>},
  >                        {routine,<<"heap_create_with_catalog">>}]}},
  >   {{create,table},[]}
  > ]
Spec fix: batch_query/{3,4,5}

A batch query might return multiple errors,
or a combination of errors and results:

  pgsql_connection:batch_query("INSERT INTO missing VALUES ($1);", [[1],[2],[3]], C)
  > [
  >   {error,{pgsql_error,[{severity,<<"ERROR">>},
  >                        {code,<<"42P01">>},
  >                        {message,<<"relation \"missing\" does not exist">>},
  >                        {position,<<"13">>},
  >                        {file,<<"parse_relation.c">>},
  >                        {line,<<"984">>},
  >   {error,{pgsql_error,[{severity,<<"ERROR">>},
  >                        {code,<<"42P01">>},
  >                        {message,<<"relation \"missing\" does not exist">>},
  >                        {position,<<"13">>},
  >                        {file,<<"parse_relation.c">>},
  >                        {line,<<"984">>},
  >   {error,{pgsql_error,[{severity,<<"ERROR">>},
  >                        {code,<<"42P01">>},
  >                        {message,<<"relation \"missing\" does not exist">>},
  >                        {position,<<"13">>},
  >                        {file,<<"parse_relation.c">>},
  >                        {line,<<"984">>}
  > ]
Merge pull request #20 from sebastian/sebastian/addition-type-fixes

Additional spec fixes