Fix ticket 1816: Database errors recorded as "Array"

Fix ticket 1816: Database errors recorded as "Array"

PEAR error backtrace lines are now correctly formatted as strings in debug log, roughly as debug_print_backtrace() does (but with argument values swapped out for types to avoid being overly verbose).

Todo: exceptions and PEAR error objects should log backtraces the same way; right now it doesn't look like exceptions get backtraces logged.

Todo: At one line per line, it's potentially tough to figure out what backtrace goes with what event if traffic is heavy; even if not heavy it's awkward to jump back into a log file after grepping to find the backtrace. Consider using a random per-event ID which can go in the log output -- bonus points for exposing the error ID to users so ops can track down actual error details in logs from a user report.