Mystikos fails to detect non-existent application

[Mike Brasher]

Mystikos does not detect when the application path does not refer to an existing executable file.

$ myst exec-sgx rootfs /bin/doesnotexist

Current behavior:

*** kernel panic: enter.c(860): myst_enter_kernel(): myst_exec() failed, ret=-2
0x1005f608e: __myst_panic()
0x1005bfd6b: myst_enter_kernel()

There is no clue indicating why the kernel panicked.

[Paul Allen]

fixed and merged