usb_attach: improve error reporting
usb_attach: improve error reporting

- specifically report when qubes-usb-proxy package is not installed
- include sanitized stderr output in error message

QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin