When starting the server using `platform server`, errors occuring during
startup are not reported in the console. The command exit with a 0 exit
code (i.e. "success"), although the server failed to launch.
With this change, when an error occurs while initializing the app (like
a missing or invalid configuration file):
- the error is printed to the console;
- the command exit with a "-1" exit code.
This allow shell scripts to properly detect the startup failure, and to
react to it.
Example of error displayed:
```
$ platform server
Error: LoadConfig: Error decoding config file=config.json, err=While parsing config: invalid character ':' after top-level value,
``
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