If __KernelLoadExec() fails, halt emulation.

Otherwise, we have no current thread, nothing can work.
sceKernelLoadExec(.../BOOT.BIN) loads EBOOT.BIN.

Fixes #2289, crash when pressing R+L+Start+Select in Final Fantasy 3.
Merge pull request #2296 from unknownbrackets/loadexec

sceKernelLoadExec(.../BOOT.BIN) should load EBOOT.BIN instead