Raise a named exception when pandoc is missing

In the normal case this will case the interpriter to exit just like
sys.exit(...), but it will make it possible for callers to explicitly
catch the exception too.

Also, removed parens from the pandoc URL so it's easier to
triple-click-copy-and-paste.

IPython/nbconvert/utils/pandoc.py

@@ -1,60 +1,71 b''
 """Utility for calling pandoc"""
 #-----------------------------------------------------------------------------
 # Copyright (c) 2013 the IPython Development Team.
 #
 # Distributed under the terms of the Modified BSD License.
 #
 # The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 
 from __future__ import print_function
 
 # Stdlib imports
-import sys
 import subprocess
 
 # IPython imports
 from IPython.utils.py3compat import cast_bytes
 
 #-----------------------------------------------------------------------------
 # Classes and functions
 #-----------------------------------------------------------------------------
 
+class PandocMissing(SystemExit):
+    """Exception raised when Pandoc is missing.
+
+    A subclass of SystemExit so it will cause an exit if not caught, but
+    explicitly named so that it's possible to catch.
+    """
+    pass
+
+
 def pandoc(source, fmt, to, extra_args=None, encoding='utf-8'):
     """Convert an input string in format `from` to format `to` via pandoc.
 
     This function will raise an error if pandoc is not installed.
     Any error messages generated by pandoc are printed to stderr.
 
     Parameters
     ----------
     source : string
       Input string, assumed to be valid format `from`.
     fmt : string
       The name of the input format (markdown, etc.)
     to : string
       The name of the output format (html, etc.)
 
     Returns
     -------
     out : unicode
       Output as returned by pandoc.
     """
     command = ['pandoc', '-f', fmt, '-t', to]
     if extra_args:
         command.extend(extra_args)
     try:
         p = subprocess.Popen(command,
                             stdin=subprocess.PIPE, stdout=subprocess.PIPE
         )
-    except OSError:
-        sys.exit("ERROR: Unable to launch pandoc. Please install pandoc:\n"
-                 "(http://johnmacfarlane.net/pandoc/installing.html)")
+    except OSError as e:
+        raise PandocMissing(
+            "Error trying to run '%s': %s.\n" %(" ".join(command), e) +
+            "Please check that pandoc is installed:\n" +
+            "http://johnmacfarlane.net/pandoc/installing.html"
+        )
     out, _ = p.communicate(cast_bytes(source, encoding))
     out = out.decode(encoding, 'replace')
     return out[:-1]