Add configuration flag to provide more verbose error output

[dr-jts]

Currently the error messages returned by the service are fairly terse, and don't give details of what went wrong. This is intentional, to avoid revealing too much about the internal database state.

For debugging purposes, or in "friendly" environments it might be useful to be able to return the internal database error messages. This could be controlled via a configuration flag. For backwards compatibility, the default mode should stay as terse mode.