function onExists( error, bool ) {
			if ( error ) {
				debug( 'Encountered an error while resolving file: %s (%d of %d). Error: %s', file, k, len, error.message );
				return clbk( error );
			}
			if ( !bool ) {
				debug( 'Unable to resolve file: %s (%d of %d). File does not exist.', file, k, len );
				error = new Error( format( 'unexpected error. File does not exist. Unable to resolve file: %s.', file ) );
				return clbk( error );
			}
			debug( 'Resolved file: %s (%d of %d). File: %s.', file, k, len, file );
			out[ i ] = file;

			count += 1;
			debug( 'Resolved %d of %d files.', count, len );
			if ( count === len ) {
				debug( 'Resolved all files.' );
				return clbk( null, out, idx );
			}
		}
Callback to be invoked upon determining if a file path exists.

@private
@param {(Error|null)} error - error object
@param {boolean} bool - boolean indicating if a file path exists
@returns {void}