refactor(docs): api: write 'Exceptions'

docs/api/exceptions.md

 # Exceptions
 
-!!! todo
-    - list exceptions available
-    - explain inheritance structure
+The library defines its own set of exceptions.
+Throughout the whole library, these are used.
+They are located in the package `de.hftstuttgart.unifiedticketing.exceptions`.
+
+Base for all others is `UnifiedticketingException`,
+which extends `RuntimeException`.
+Every other custom exception extends this exception.
+
+This provides you with the ability,
+to catch a specific or all library related problems.
+Both without catching exceptions of your own code,
+that happen to be in the same try block.
+
+## Possible Exceptions
+
+- `AssertionException`
+- `DeserializationException`
+- `HttpRequestException`
+- `HttpResponseException`
+- `SerializationException`
+- `UnsupportedFunctionException`