test-umockdev: Check uevents for subsystem with long name

Change gudev uevent test to use subsystem "bluetooth" instead of "usb" to
trigger the 4-byte block string hash code in uevent_sender.c.
test-umockdev: Check uevent for nonexisting device
load_socket_script(): Throw exception instead of assertion for invalid sockets
umockdev-{run,record}: Simplify exception handling

Where applicable, just catch "Error" instead of a specific one, to avoid
potentially ugly output if we catch a different one and simplify the generated
C code.