Improve the behaviour if pandoc isn't in the path

[claremacrae]

This is the output, if pandoc isn't in the user's path:

'pandoc' is not recognized as an internal or external command,
operable program or batch file.
Traceback (most recent call last):
  File "markdown_to_dokuwiki.py", line 128, in <module>
    main( files )
  File "markdown_to_dokuwiki.py", line 119, in main
    convert_file( filename )
  File "markdown_to_dokuwiki.py", line 109, in convert_file
    input_file = open(tempfile, 'r' )
IOError: [Errno 2] No such file or directory: 'temp_script_output.json'

It would be better if the script recognised this, and produced a useful message, rather than carrying on and trying to read a non-existent output file.

This question provides ways to do that:

http://stackoverflow.com/questions/377017/test-if-executable-exists-in-python

And this answer looks especially useful:

http://stackoverflow.com/a/12611523/104370

[claremacrae]

It would be good if any fix ensured that the return code from main was 1, indicating failure.

[claremacrae]

Now, if pandoc isn't in the path, the testsuite output looks like this:

'pandoc' is not recognized as an internal or external command,
operable program or batch file.
No json file created: cannot proceed
F
======================================================================
FAIL: runTest (__main__.ConversionsTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "markdown_to_dokuwiki_testsuite.py", line 29, in runTest
    self.fail( "ERROR: conversion failed." )
AssertionError: ERROR: conversion failed.

----------------------------------------------------------------------
Ran 1 test in 0.092s

FAILED (failures=1)

I think this is clear enough to close the issue.