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`ERR` is not a signal, so there is no signal code to get. From `man bash`:

> If a sigspec is ERR, the command arg is executed whenever a pipeline (which may consist of a single simple command), a list, or a  compound  command returns  a  non-zero exit status, subject to the following conditions. The ERR trap is not executed if the failed command is part of the command list  immediately  following a while or until keyword, part of the test in an if statement, part of a command executed in a && or || list except the  command  following the final && or ||, any command in a pipeline but the last, or if the command's return value is being inverted using !.  These are the same conditions obeyed by the errexit (-e) option.