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,
``

cmd/platform/server.go

@@ -31,6 +31,7 @@ var serverCmd = &cobra.Command{
 	Use:          "server",
 	Short:        "Run the Mattermost server",
 	RunE:         runServerCmd,
+	SilenceUsage: true,
 }
 
 func runServerCmd(cmd *cobra.Command, args []string) error {
@@ -41,11 +42,10 @@ func runServerCmd(cmd *cobra.Command, args []string) error {
 
 	disableConfigWatch, _ := cmd.Flags().GetBool("disableconfigwatch")
 
-	runServer(config, disableConfigWatch)
+	return runServer(config, disableConfigWatch)
-	return nil
 }
 
-func runServer(configFileLocation string, disableConfigWatch bool) {
+func runServer(configFileLocation string, disableConfigWatch bool) error {
 	options := []app.Option{app.ConfigFile(configFileLocation)}
 	if disableConfigWatch {
 		options = append(options, app.DisableConfigWatch)
@@ -54,7 +54,7 @@ func runServer(configFileLocation string, disableConfigWatch bool) {
 	a, err := app.New(options...)
 	if err != nil {
 		l4g.Error(err.Error())
-		return
+		return err
 	}
 	defer a.Shutdown()
 
@@ -172,6 +172,8 @@ func runServer(configFileLocation string, disableConfigWatch bool) {
 
 	a.Jobs.StopSchedulers()
 	a.Jobs.StopWorkers()
+
+	return nil
 }
 
 func runSecurityJob(a *app.App) {