The textual description of the last
server-related error. Returns 0
if no error has occurred
yet. It is not cleared upon reading (can be invoked multiple
times, will return the same result until a new error occurs).
During the execution of a statement, this function accumulates all
non-error messages (notices, warnings, etc.). If a statement does not
generate any errors, this function will return all collected messages
from the last statement.