first_stage_console: Fix execv() child error path

If the /first_stage.sh is not present on the system, the child process
which attempted to execv() ends up returning from the function along
with its parent, which can be seen in early logs e.g.

[   10.747576][   T51] init: Attempting to run /first_stage.sh...
[   10.757371][   T52] init: unable to execv /first_stage.sh, returned -1 errno 2
[   10.767527][   T52] init: unable to execv, returned -1 errno 2
                      [...]
[   10.789189][   T51] init: unable to execv, returned -1 errno 2

where both T51 and T52 end up executing the "rest" of StartConsole().

Instead, terminate the child if its execv() failed.

Test: run first_stage_init
Change-Id: I20bc0aeae627761a60fb2b55bae39871ad506f69