fix: replace bare std::exception throws with descriptive place holder…

[e-hatch]

replace bare std::exception throws with placeholders

Changes

• verify() now throws runtime_error with the SQLite error string via sqlite3_errstr(rc)
• Iterator::operator++() throws runtime_error via sqlite3_errmsg(sqlite3_db_handle(stmt_)) on unexpected step
• Iterator::operator++() also throws std::logic_error on null statement handle