Handle signals of code-blocks by returning an error as the result

During the execution of code blocks an error could be signaled.  The
consequence of this would be that the FINISH-FUNC will not be called.
Therefore handle the error in the START-FUNC and return a textual
representation of it instead of the result.

A code block like the following

```
  #+begin_src emacs-lisp :async
  (error "somewhere an error was signaled")
  #end_src
```

would no longer lead to a #+result that gets never updated and keeps
the UUID forever.  Instead the following would be put as the result:

  Failure: during the async execution of your code block the following error was signaled:
    error: (somewhere an error was signaled)

ob-async.el

@@ -169,7 +169,12 @@ block."
                       (let ((default-directory ,default-directory))
                         (with-temp-buffer
                           (insert org-babel-async-content)
-                          (,cmd ,body ',params))))
+                          (condition-case context
+                              (,cmd ,body ',params)
+                            (error (format
+                                    "Failure: during the async execution of your code block the following error was signaled:\n   %s: %s"
+                                    (car context)
+                                    (cdr context)))))))
                    `(lambda (result)
                       (with-current-buffer ,(current-buffer)
                         (let ((default-directory ,default-directory))