pldmtool: Exit non-zero on request failures

exec()'s three failure paths (instance ID allocation,
createRequestMsg(), pldmSendRecv()) now report failure through the CLI
framework's exit-code mechanism instead of an uncaught exception or a
plain return.

Motivation:
pldmtool surfaces the instance ID allocation failure as an exception
rather than silently discarding it. Nothing above exec() catches that
exception, so a real, expected failure (instance IDs running out under
normal contention, not misuse) crashes the whole process instead of
exiting cleanly. The other two paths avoid that crash, but log an error
and still return successfully as far as the caller can tell, so a script
checking the exit status cannot distinguish a failed request from a
successful one.

Tested:
Forced each of the three failure paths above on Yosemite4.

Instance ID pool forced empty, before:
```
$ pldmtool base GetTID -m 30
<3> Instance ID allocation failed for EID 30: Failed to allocate
instance ID for EID 30: No free instance ids
terminate called after throwing an instance of 'pldm::InstanceIdError'
  what():  Failed to allocate instance ID for EID 30: No free
  instance ids
Aborted (core dumped)
$ echo $?
134
```
after:
```
$ pldmtool base GetTID -m 30
<3> Instance ID allocation failed for EID 30: Failed to allocate
instance ID for EID 30: No free instance ids
$ echo $?
1
```

createRequestMsg() forced to return an encode-validation error, before:
```
$ pldmtool base GetTID -m 30
Failed to encode request message for base:GetTID rc = 2
$ echo $?
0
```
after:
```
$ pldmtool base GetTID -m 30
Failed to encode request message for base:GetTID rc = 2
$ echo $?
1
```

pldmSendRecv() forced to return a transport send error, before:
```
$ pldmtool base GetTID -m 30
pldmSendRecv: Failed to receive RC = -7
$ echo $?
0
```
after:
```
$ pldmtool base GetTID -m 30
pldmSendRecv: Failed to receive RC = -7
$ echo $?
1
```

Change-Id: Ifbc0235b431e50df104322d3d5d156df383a8aa2
Signed-off-by: Eric Yang <eric.yang.wiwynn@gmail.com>