o-b.org: added shell exit code error checking task

org-babel.org

@@ -722,6 +722,25 @@ should use a span class, and should show original source in tool-tip
 ** TODO LoB: re-implement plotting and analysis functions from org-R
    I'll do this soon, now that we things are a bit more settled and we
    have column names in R.
+** TODO Improved error checking
+   E.g. when trying to execute sass block, I did not have sass
+   installed, and so shell-command returned code 127, but org-babel
+   did not warn me that anything had gone wrong.
+*** DEFERRED figure out how to handle errors during evaluation
+    I expect it will be hard to do this properly, but ultimately it
+    would be nice to be able to specify somewhere to receive STDERR,
+    and to be warned if it is non-empty.
+
+    Probably simpler in non-session evaluation than session? At least
+    the mechanism will be different I guess.
+
+    R has a try function, with error handling, along the lines of
+    python. I bet ruby does too. Maybe more of an issue for functional
+    style; in my proposed scripting style the error just gets dumped to
+    the org buffer and the user is thus alerted.
+
+    For now I think the current behavior of returning any error
+    messages generated by the source language is sufficient.
 ** PROPOSED allow `anonymous' function block with function call args?
    My question here is simply whether we're going to allow
 #+begin_src python(arg=ref)
@@ -820,21 +839,6 @@ manner.
      update the source-code blocks timestamp.  If the user edits the
      contents of a source-code block directly I can think of no
      efficient way of maintaining the timestamp.
-** DEFERRED figure out how to handle errors during evaluation
-   I expect it will be hard to do this properly, but ultimately it
-   would be nice to be able to specify somewhere to receive STDERR,
-   and to be warned if it is non-empty.
-
-   Probably simpler in non-session evaluation than session? At least
-   the mechanism will be different I guess.
-
-   R has a try function, with error handling, along the lines of
-   python. I bet ruby does too. Maybe more of an issue for functional
-   style; in my proposed scripting style the error just gets dumped to
-   the org buffer and the user is thus alerted.
-
-   For now I think the current behavior of returning any error
-   messages generated by the source language is sufficient.
 ** DEFERRED source-name visible in LaTeX and html exports
 Maybe this should be done in backend specific manners.