Improve formatting of SQLite errors in Hrana

[honzasp]

SQLite input error (statement SELECT foo) is now reported as:

{
  "message": "SQL input error: no such column: foo (at offset 7)",
  "code": "SQL_INPUT_ERROR"
}

and other SQLite errors (such as constrait violation) is now reported as:

{
  "message": "SQLite error: UNIQUE constraint failed: t.a",
  "code": "SQLITE_CONSTRAINT"
}

Fixes #390.