Gracefully handle startup errors

[andig]

If startup fails, show GUI for 5 minutes, then restart. Fatal error messages (fatal) will be displayed to aid new users with diagnosis.

Todo

• differentiate no config (show demo) vs broken config (show fatal error)
• expose fatal gui (@naltatis)
• rework configured check (@naltatis)
• shutdown endpoint -> POST /api/shutdown (@andig)
• translation texts (@naltatis)
• show anonymised config

Refs spf13/viper#1433, go-yaml/yaml#760

[naltatis]

yaml parsing errors should also result in readable error message, currently the process exists on invalid yamls.

Example:

While parsing config: yaml: line 57: did not find expected '-' indicator

[andig]

yaml parsing errors should also result in readable error message, currently the process exists on invalid yamls.

Gute Idee! Done.

[naltatis]

Nicer design for error page.

@andig The restart button calls /api/shutdown which does not exist yet.

[andig]

Der PR enthält komische Dinge- hast du neAhnung, wieso?

[naltatis]

[premultiply]

Könnte man in der Darstellung der Konfigdatei noch die Zeilennummern vorne mit angegeben?