Apple doesn't provide much information to the end user when the installation goes wrong. Often, error messages are generic ones like "The application could not be downloaded".
One way to get a little bit more information is to connect the device to a Mac with xcode, and look at the console.
The device console is located in xcode's Window > Devices menu
In the Device window, choose your device on the left and the console will appear in the bottom right.
Search for lines containing itunesstored: those error messages often help debugging problems with installations.
- Cannot load non-https manifest URL
The error in the screenshot above means that a manifest plist was requested on an HTTP url, but Apple requires that manifests must be served through HTTPS since iOS 8+.
This error should never happen on Diawi, but may happen on custom setups.