algokey 'succeeds' even when it doesn't actually sign a transaction

### Subject of the issue

`algokey multisig` returns exit code 0 even when it doesn't actually sign a transaction

### Your environment

MacOS terminal

### Steps to reproduce

```
% algokey generate
Private key mnemonic: entire join unknown puzzle before crumble torch tumble own net river cloud oval decline insect edit cute dress negative clarify subway seminar warfare absent describe
Public key: ESKN72RO7Z5N2JBWVYA2DAUQ45MIKVPEZPDQ4PSPGLTA5KR2H72BYMPHZI
% touch empty-file.txn
% algokey multisig -m "entire join unknown puzzle before crumble torch tumble own net river cloud oval decline insect edit cute dress negative clarify subway seminar warfare absent describe" -t empty-file.txn -o empty-file.stxn
%
```

### Expected behaviour
If algokey is unable to sign a given input file it should throw an error.

### Actual behaviour
The transaction file was empty so it had nothing to sign, yet it still produced an empty file and returned successful.