simulator-run.sh: Don't try to execute the simulator if doesn't exist.
simulator-run.sh: Don't try to execute the simulator if doesn't exist.

Getting a success message when the script failes to execute the
simulator is really annoying. Better to get a descriptive error message.

Signed-off-by: Philip Tricca <fd418754d31777cf39b16b99b0350486ab840079@intel.com>

01org/TPM2.0-TSS,01org/TPM2.0-TSS,01org/tpm2-tss,tpm2-software/tpm2-tss,tpm2-software/tpm2-tss,01org/tpm2-tss,01org/TPM2.0-TSS,01org/TPM2.0-TSS,tpm2-software/tpm2-tss,01org/tpm2-tss