Url validation before sharing in Share Plugin

Summary

While using CLI version of KDE connect, one can use the following command: ./kdeconnect-cli -n XXXX --share /home/sandy/ The url is Invalid since it is not a file. The sharing should not initiate.

Fixes: While using command: ./kdeconnect-cli -n XXXX --share /home/sandy/ Daemon will Prompt that "/home/sandy is not a file" And sharing will not be initiated.

##Test Plan: Run following command with XXXX->Device_name :

./kdeconnect-cli -n XXXX --share /home/sandy/

Before:

Sharing starts.

After:

You get a prompt that "/home/sandy is not a file"