Make add module error message more verbose

[AlenPelin]

"This is not a Sitecore module" while trying to add a Sitecore package to the modules to be added on top of Sitecore - this should be described in detail.

[DmitryKolinchuk]

The "Selected file is not a Sitecore module package" message is defined in the "SIM.Tool.Windows.UserControls.Install.Modules.ModulesDetails.CustomButtonClick" method that is used when installing a package to Sitecore 9 and later, so this feature potentially can be considered for implementation.