function processExit( code, signal ) {
		if ( err ) {
			return;
		}
		if ( code !== null && code !== 0 ) {
			err = new Error( cmd+' '+script+' failed with exit code: '+code+'.' );
		} else if ( signal !== null ) {
			err = new Error( cmd+' '+script+' failed due to termination signal: '+signal+'.' );
		}
		if ( err ) {
			err.code = code;
			err.signal = signal;
		}
	}
Processes process exit values and sets the `err` state.

@private
@param {(number|null)} code - exit code
@param {(string|null)} signal - termination signal