If there is no adb, converting misleading fatal error to a warning

In the current system, if there is no adb on the system (an android tool), it builds everything normally, but gives a seemingly fatal error:

Created pkg/https-everywhere-5.0development.2~0e77731-dirty.xpi
./makexpi.sh: Zeile 175: type: adb: Not found.

It is probably not the intended behavior. This changes the misleading fatal error to a warning:

Created pkg/https-everywhere-5.0development.2~0e77731-dirty.xpi
Warning: adb not found, no android version