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.)