Feature/669 show errors watch ui

Describe the MR

Adds a notification system to panto•watch as a global Vue plugin.

Why was this MR needed?

Errors were silently ignored.

Extra-care points?

Use in components by calling

this.$notification({
  title: 'A title',
  color: 'danger',
  timeout: 2000
});

All fields are optional.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• CHANGELOG updated
• Unit Tests added/updated

What are the relevant issue numbers?

Closes #669 (closed)

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