report server launch errors (#8189)

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,
``

0 files changed, 0 insertions, 0 deletions