recovery: additional checks before starting installation

[amartinz]

@amartinz we need a check in the installer if cache partition is writable before installer pushes files.

Originally posted by @Flohack74 in https://github.com/ubports/ubports-installer/issues/2806#issuecomment-1222348368

[Flohack74]

• userdata mountable
• userdata not encrypted (push a test file and see if it comes back correctly)
• cache mounted and writable

Those would solve 20% of installation errors right away