function onRead( error, data ) {
			if ( error ) {
				debug( 'Encountered an error while attempting to read file: %s. Error: %s.', files[ idx ], error.message );
				return clbk( error );
			}
			count += 1;

			debug( 'Successfully read file: %s.', files[ idx ] );
			out[ files[ idx ] ] = data.toString();

			debug( 'Read %d of %d files.', count, files.length );
			if ( count === files.length ) {
				debug( 'Finished reading files.' );
				clbk( null, out );
			}
		}
Callback to be invoked upon reading a file.

@private
@param {(Error|null)} error - error object
@param {(Buffer|string)} data - file content
@returns {void}