Make compatible with Octaves new exception-based error handling. Retain compatibility with Octaves old error handling based on error_state. * src/error_helpers.[h,cc]: Added. * src/Makefile.in: Integrate error-helpers.[h,cc]. * src/config.h.in: Added. * configure.ac, src/config.h.in: Test presence of 'error_state' and presence of 'verror (octave_execution_exception&, const char *, va_list)'. * src/__pq_connect__.cc, src/command.cc, src/command.h, src/converters.cc, src/converters_arr_comp.cc, src/pq_connection.cc, src/pq_conninfo.cc, src/pq_exec.cc, src/pq_lo.cc, src/pq_update_types.cc: If necessary, include error-helpers.h, replace error() with c_verror(), set and check a different error indicator than error_state, use CHECK_ERROR or SET_ERR, explicitely check for errors instead of relying on Octave checking error_state when returning to the prompt.