Document changes to ob-ein. Error when no :image header specified.

ob-ein will alert the user if they are generating an image in the ob-ein source
block and no :image header has been supplied. Added some documentation for good
measure, though people will probably still get tripped up by this.

doc/source/index.rst

@@ -369,6 +369,22 @@ argument. The format for the session argument is
    sys.version
    #+END_SRC
 
+If your code block generates an image, like from an matplotlib plot, then
+specify the file to save the image to using the `:image` argument as in the
+example below:
+
+.. code:: python
+
+   #BEGIN_SRC ein :session 8888/Untitled.ipynb
+   import matplotlib.pyplot as plt
+   import numpy as np
+
+   %matplotlib inline
+   x = np.linspace(0, 1, 100)
+   y = np.random.rand(100,1)
+   plt.plot(x,y)
+   #+END_SRC
+
 You can also link to an IPython notebook from org-mode_ files.
 
 1. Call org-mode function :el:symbol:`org-store-link`

lisp/ob-ein.el

@@ -66,10 +66,14 @@
    (case key
      ((svg image/svg)
       (progn
+        (when (null file)
+          (error "Please specify an :image header argument when generating images."))
         (ein:write-base64-decoded-image value file)
         (format "[[file:%s]]" file)))
      ((png image/png jpeg image/jpeg)
       (progn
+        (when (null file)
+          (error "Please specify an :image header argument when generating images."))
         (ein:write-base64-image value file)
         (format "[[file:%s]]" file)))
      (t (plist-get json type)))))