#1497 Show specific message for invalid YAML files (#1506)

* #1497 Throw dedicated exception for invalid YAML files and handle it on startup
- Wrap SnakeYAML exceptions when loading config.yml and commands.yml on startup into own exception type
- Handle exception type on startup with specific error message

* #1497 Fix inaccurate JavaDoc comment

src/test/resources/fr/xephi/authme/service/yaml/invalidYaml.yml

@@ -0,0 +1,5 @@
+# File with invalid YAML
+test:
+  abc: 'test'
+  def: 'Going to forget a quote here
+  jkl: 'Test test'

src/test/resources/fr/xephi/authme/service/yaml/validYaml.yml

@@ -0,0 +1,4 @@
+test:
+  abc: 'test'
+  def: 'All quotes are good here'
+  jkl: 'Test test'