Notify when no datasets are available for download

[joverlee521]

Prompted by nextstrain/auspice.us#93

The download Auspice JSON feature depends on fetching the original JSON by parsing the pathname.
It silently skips invalid datasets so users are not notified if the dataset is not available:

auspice/src/components/download/helperFunctions.js

Lines 634 to 635 in 835fbb2

	for (const datasetName of getDatasetNamesFromUrl(window.location.pathname)) {
	if (!datasetName) continue; // e.g. no 2nd tree

Possible solutions

1. Dispatch error notification that the dataset is not available for download if all dataset names are invalid.
2. Hide the download Auspice JSON button if we cannot parse the dataset name from the URL pathname.