Query.Exec: Log error when talking to job process fails

Query.Exec: Log error when talking to job process fails

When the forked (Python) job process dies for any reason, then the next `read`
from its pipe will fail, giving an unhelpful error like:

   GenericError "<file descriptor: 13>: hWaitForInput: end of file"

This commit adds an error log message when reading or writing the
pipe fails.

Signed-off-by: Niklas Hambuechen <niklash@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>
Signed-off-by: Klaus Aehlig <aehlig@google.com>

Conflicts:
	src/Ganeti/Query/Exec.hs - merge changes from both branches

Cherry-picked-from: 208c6166d9176e36495ce3d8dff9a649662e72f5
Signed-off-by: Petr Pudlak <pudlak@google.com>
Signed-off-by: Klaus Aehlig <aehlig@google.com>