shell: add string fields for debugging

src/shell.js

@@ -81,8 +81,10 @@ function spawn(tag, file, args, options) {
 
             const e = new BoxError(BoxError.SHELL_ERROR, `${file} exited with code ${code} signal ${signal}`);
             e.stdout = stdout; // when promisified, this is the way to get stdout
+            e.stdoutString = options.encoding ? stdout : stdout.toString(); // make error printable
             e.stdoutLineCount = stdoutLineCount;
             e.stderr = stderr; // when promisified, this is the way to get stderr
+            e.stderrString = options.encoding ? stderr : stderr.toString(); // make error printable
             e.stderrLineCount = stderrLineCount;
             e.code = code;
             e.signal = signal;