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_ftsmock.go

@@ -6,8 +6,8 @@ import "reflect"
 const JSCKSourceFTSMock string = "fts_mock"
 
 // JSCSourceFTSMock configures fetching from a Find a Tender mock
-// server (cmd/mock-fts) at APIRoot, which has no default and must be set when
-// the source is enabled.
+// server (cmd/mock-fts) at APIRoot, which has no default. Empty: the source
+// fails to start if it is enabled.
 type JSCSourceFTSMock struct {
 	JSCSourcesSource
 	APIRoot string `cfg:"api_root"`