Fallback to the string printer if the latex printer raises an exception

Currently it just prints an error and gives an empty png. However, there is a
lot of LaTeX that matplotlib cannot parse, e.g., matrices. This gives a better
user experience for SymPy, which has a text printer that it can fallback
to. SymPy cannot just disable the LaTeX printer because it is desired for the
notebook, and there is no way for it to enable LaTeX just in the notebook, or
change the printer precedence order.

I have left in the debug logging of the error (to the terminal).

Fixes jupyter#63. This supersedes jupyter#57, although I don't know if it fixes the issue
there.

qtconsole/rich_jupyter_widget.py

@@ -138,7 +138,12 @@ def _handle_execute_result(self, msg):
                 self._append_html(self.output_sep2, True)
             elif 'text/latex' in data:
                 self._pre_image_append(msg, prompt_number)
-                self._append_latex(data['text/latex'], True)
+                try:
+                    self._append_latex(data['text/latex'], True)
+                except Exception as e:
+                    self.log.error("Failed to render latex: '%s'", data['text/latex'], exc_info=True)
+                    self.log.error("Failed to render latex: %s" % e)
+                    return super(RichJupyterWidget, self)._handle_execute_result(msg)
                 self._append_html(self.output_sep2, True)
             else:
                 # Default back to the plain text representation.
@@ -166,7 +171,12 @@ def _handle_display_data(self, msg):
                 jpg = decodestring(data['image/jpeg'].encode('ascii'))
                 self._append_jpg(jpg, True, metadata=metadata.get('image/jpeg', None))
             elif 'text/latex' in data and latex_to_png:
-                self._append_latex(data['text/latex'], True)
+                try:
+                    self._append_latex(data['text/latex'], True)
+                    self.log.error("Failed to render latex: '%s'", data['text/latex'], exc_info=True)
+                    self.log.error("Failed to render latex: %s" % e)
+                except Exception as e:
+                    return super(RichJupyterWidget, self)._handle_display_data(msg)
             else:
                 # Default back to the plain text representation.
                 return super(RichJupyterWidget, self)._handle_display_data(msg)
@@ -177,13 +187,7 @@ def _handle_display_data(self, msg):
 
     def _append_latex(self, latex, before_prompt=False, metadata=None):
         """ Append latex data to the widget."""
-        try:
-            png = latex_to_png(latex, wrap=False)
-        except Exception as e:
-            self.log.error("Failed to render latex: '%s'", latex, exc_info=True)
-            self._append_plain_text("Failed to render latex: %s" % e, before_prompt)
-        else:
-            self._append_png(png, before_prompt, metadata)
+        self._append_png(latex_to_png(latex, wrap=False), before_prompt, metadata)
 
     def _append_jpg(self, jpg, before_prompt=False, metadata=None):
         """ Append raw JPG data to the widget."""
@@ -365,4 +369,3 @@ class RichIPythonWidget(RichJupyterWidget):
     def __init__(self, *a, **kw):
         warn("RichIPythonWidget is deprecated, use RichJupyterWidget")
         super(RichIPythonWidget, self).__init__(*a, **kw)
-