Annotate internal_error with the attribute noreturn

Annotating functions that do not return with the noreturn attribute
(which is understood by both gcc and clang) prevents static analyzers
from generating false positives (internal_error is used to terminate
the process and is used extensively in error handling code paths).

Remove the return statement that was placed there to appease the
compiler. Functions annotated with noreturn are not supposed to return
any values.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>