tmplcvt: make the tool more robust

Currently, the script just assumes to be called at
Documentation/sphinx/. Change it to work on any directory,
and make it abort if something gets wrong.

Also, be sure that both parameters are specified.

That should avoid troubles like this:

$ Documentation/sphinx/tmplcvt Documentation/DocBook/writing_usb_driver.tmpl
sed: couldn't open file convert_template.sed: No such file or directory

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>