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

@@ -14,7 +14,6 b''
 from __future__ import print_function
 
 # Stdlib imports
-import sys
 import subprocess
 
 # IPython imports
@@ -24,6 +23,15 b' 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.
 
@@ -51,9 +59,12 b" def pandoc(source, fmt, to, extra_args=None, encoding='utf-8'):"
         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]