enhancement: better messaging for K6 not installed

[askalik]

Improve the message for when the k6 binary isn't found in the PATH, used try, except to handle exception.

I think this should resolve issue #4

[Lawouach]

Thanks @askalik

I think this time I prefer asking permission than forgiveness. So I would have added the following snippet here:

    path = shutil.which('k6')
    if not path:
        raise ActivityFailed("the 'k6' binary cannot be found")

It uses this module. Then use the path variable in the subprocess call to use the absolute path of the k6 binary.

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