Write failed watcher init attempts to log file

Summary:
I have seen Watchman use the inotify watcher instead of the edenfs watcher for an EdenFS (FUSE) repository. Unfortunately, Watchman didn't report why it couldn't use the edenfs watcher.

Whenever Watchman falls back to a lower-quality watcher (such as kqueue on macOS), log what the problem with the higher-quality watcher was.

Reviewed By: wez

Differential Revision: D12742390

fbshipit-source-id: 935031f66ee5be2d41b098b6ce7e34f6a2a32ca0

watcher/auto.cpp

@@ -117,7 +117,13 @@ std::shared_ptr<watchman::QueryableView> WatcherRegistry::initWatcher(
       // in this case
       break;
     } catch (const std::exception& e) {
-      watchman::log(watchman::DBG, watcher->getName(), ": ", e.what(), ".\n");
+      watchman::log(
+          watchman::ERR,
+          "failed to use watcher ",
+          watcher->getName(),
+          ": ",
+          e.what(),
+          ".\n");
       failureReasons.append(
           watcher->getName() + std::string(": ") + e.what() +
           std::string(". "));