give more informative error when we can't base64 decode something

I was trying to spin up an internal service with an PAL_SECRETS_YAML that had files that were base64 encoded:
```
PAL_SECRETS_YAML: |
prod:
file:
/tmp/ca.pem.64: %DATACA%
```

Where %DATACA% was replaced by the output of `base64`. That output was erroneous in that it had spaces and PAL refused to parse it on startup. The error message was pretty unhelpful:
```
2017-07-18T14:47:04.455 host base64: invalid input
```

I would prefer an error message that indicated that this error came from PAL and which value in the env caused the error in parsing.