plugin ext_tools: check if script file exists

[bastiK]

To avoid problems due to typos, the plugin should:

• create the script folder plugins\ext_tools by default

• check if the script file exists and give a warning if not

[bastiK]

Some documentation for the output format would be nice.

[Upliner]

1) Plugin already creates plugins/ext_tools directory by default. Actually it is only needed as working directory for scripts, actually one can place scripts anywhere he want and call them using full path.
2) It's too difficult to bother because script can be called directly "./script.sh", indirectly "python /path/to/script.py" or on some more complicated way: 'sh -c "python script1.py | python script2.py"'.
3) Output format is regular .osm file. I added this information to the wiki.

[Upliner]

• "create the script folder plugins\ext_tools by default" -> worksforme

• "check if the script file exists and give a warning if not" -> partially fixed in ​[o23327], will now give an error if script cannot be executed.