Fix exit and crash in kio_file when put() fails in readData.

This strange call to ::exit() has been there forever (since the
make_it_cool branch merge in 2000), but the recently added LegacyCodec
global object leads to a crash (mem corruption says glibc) when this
brutal exit happens. Let's just not exit, but handle the error properly.

Testcase: jobtest asyncStoredPutReadyReadAfterFinish