An empty api_root now fails at start, for an enabled find_a_tender or fts_mock source and for fetch once

An enabled Find a Tender or mock source with no api_root used to start
and then fail on every fetch. Its worker now returns an error naming the
missing key before it starts, which stops Run. "fetch once" checks
sources.find_a_tender.api_root the same way. TestRunMissingAPIRoot covers
both sources, and TestRunLogsSourceErrors sets an api_root so it still
tests the zero interval. The section docs and internal/config/README.md
say what an empty api_root means.

internal/config/conf_source_findatender.go

@@ -6,7 +6,8 @@ import "reflect"
 const JSCKSourceFindATender string = "find_a_tender"
 
 // JSCSourceFindATender configures fetching from the Find a Tender
-// OCDS API at APIRoot.
+// OCDS API at APIRoot. Empty: the source fails to start if it is enabled,
+// and `fetch once` fails.
 type JSCSourceFindATender struct {
 	JSCSourcesSource
 	APIRoot string `cfg:"api_root" default:"https://www.find-tender.service.gov.uk/api/1.0"`