Backport PR #9203: Latextools: Make latex_to_png_mpl not fail on errors

- Also make make both `latex_to_foo` functions to return None on failure.
- This will help to solve the errors qtconsole is having to handle Latex (as reported in jupyter/qtconsole#56)

IPython/lib/latextools.py

@@ -97,6 +97,7 def latex_to_png(s, encode=False, backend=None, wrap=False):
 def latex_to_png_mpl(s, wrap):
     try:
         from matplotlib import mathtext
+        from pyparsing import ParseFatalException
     except ImportError:
         return None
     
@@ -105,10 +106,13 def latex_to_png_mpl(s, wrap):
     if wrap:
         s = u'${0}$'.format(s)
     
+    try:
     mt = mathtext.MathTextParser('bitmap')
     f = BytesIO()
     mt.to_png(f, s, fontsize=12)
     return f.getvalue()
+    except (ValueError, RuntimeError, ParseFatalException):
+        return None
 
 
 def latex_to_png_dvipng(s, wrap):
@@ -138,6 +142,8 def latex_to_png_dvipng(s, wrap):
 
         with open(outfile, "rb") as f:
             return f.read()
+    except subprocess.CalledProcessError:
+        return None
     finally:
         shutil.rmtree(workdir)