exiftool, Check whether exiftool is installed in the host system

[dgrammlich]

• refs Dependency on exiftool binary is not checked #2

[ramack]

Thanks a lot for your contribution! You PR will finally hide the fact that exiftool binary was not found on the server, in which case the use of the plugin is a bit strange from the start. By hiding the error I fear, that it might even trigger more questions, because it will not be visible - not even the administrator - to see that it will never do anything useful.

So I think merging this makes sense as soon as we have a a message in the admin panel.

[ramack]

ah BTW, did you take a look at #5? which solution do you prefer? But anyhow this also doesn't solve the problem that the server admin might install the plugin without it doing something useful ever without any warning...