Demolish useError

Requires !1042 (merged), closes #226 (closed)

• Removed the error slot from the paginator
• Converted these actions to notify and added unit tests:
  • image/listElements
  • classification/listCorpusMlClasses
  • classification/validate
  • classification/reject
  • entity/get
  • entity/listElements
  • entity/listInTranscription
• Moved all 4 "helpers" from js/helpers/metadata to store actions, rewrote them with notify and added unit tests
• Removed state.error from vue/Element/Main, vue/Element/Transcription/Box, vue/Entity/Elements, vue/Image/Elements
• Rewrote error handling in vue/Element/Metadata/Metadata and vue/Entity/Details

Side note: the vue/Element/Metadata/Metadata.vue component is really hard to read and should definitely be split into multiple components.