All the glue functions that interface to the database check for
error conditions. An error condition that arises is manifest
as the signalling of a <pg95-error>, which is a kind of
<error>.
Most functions will signal a <pg95-exec-error, which indicates
an error in the execution of a query or command. The other case is
a <pg95-connect-error>, which denotes an error attempting to
connect the the database. (The reason for the distinction is that
in the latter case, a connection exists and can be used to identify
the error. In the latter case, no valid connection exists.)