Further improved behaviour, if pandoc not in path

The previous commit didn't actually return a non-zero result if pandoc
didn't exist.

Now the code does, and any error message is written to stderr.

This improves the behaviour for issue #4
Improve the testsuite, if pandoc is not in path

The testsuite now checks the return code from the conversion, and if
there was an error, it doesn't try to check the contents of an empty
output file.

This completes work on issue #4