Name the setting and value when a size does not parse (closes #429)

A size setting such as chunk_size: banana failed with "invalid size
format: strconv.ParseFloat: parsing "": invalid syntax", naming neither
the setting nor the value. The size decoder now quotes the value, and
Parse checks every size setting after ${...} references are filled in,
so the error reads chunk_size: "banana" is not a size; use a number of
bytes or a unit such as 10MB. This covers chunk_size, blob_size_limit
and s3.part_size, and config set.

Model: opus-5-5

internal/cli/config_test.go

@@ -252,7 +252,7 @@ func TestConfigSetKeepsFileThatWouldNotLoad(t *testing.T) {
 		{configWithoutRecipients, "snapshots.home.paths", "/srv",
 			"into []string"},
 		{configWithoutRecipients, "chunk_size", "fast",
-			"invalid size format"},
+			`chunk_size: "fast" is not a size`},
 		// Values of the right type that fail validation.
 		{configWithoutRecipients, "compression_level", "99",
 			"compression_level must be between 1 and 19"},