sync: handle TOML parsing exception for metadata.toml

[ee7]

Follow-up from #614.

configlet sync will produce an unhandled exception if the user's cached prob-specs dir contains an invalid metadata.toml file, or if its values are of the wrong type.

configlet/src/sync/sync_metadata.nim

Lines 21 to 32 in 5a2937d

	proc parseMetadataToml(path: string): UpstreamMetadata =
	## Parses the problem-specifications `metadata.toml` file at `path`, and
	## returns an object containing the `blurb`, `source`, and `source_url` values.
	let t = parsetoml.parseFile(path)
	result = UpstreamMetadata(
	blurb:
	if t.hasKey("blurb"): t["blurb"].getStr() else: "",
	source:
	if t.hasKey("source"): t["source"].getStr().some() else: none(string),
	source_url:
	if t.hasKey("source_url"): t["source_url"].getStr().some() else: none(string)
	)

But it's unlikely that anybody notices: the unhandled exception should only occur if a user has manually edited a metadata.toml file in their prob-specs cache to contain invalid TOML, and we have some protection against invalid prob-specs cache states.

However, the prob-specs CI doesn't currently lint metadata.toml files, so we do currently depend on prob-specs reviewers for the metadata.toml files to be valid upstream. I've opened exercism/problem-specifications#2061 to track that.