Make file-util error messages more useful.

 * Canonicalize one instance of creating an error message from fs::error
 * Filesystem messages: if there is an empty filename, explicitly show that.
 * UpwardFileSearch: provide more details in the emitted status.

common/util/file_util.cc

@@ -79,9 +79,8 @@ static absl::Status CreateErrorStatusFromSysError(absl::string_view filename,
                                                   const char *fallback_msg) {
   const char *const system_msg =
       sys_error == 0 ? fallback_msg : strerror(sys_error);
-  const std::string msg = filename.empty()
-                              ? std::string{system_msg}
-                              : absl::StrCat(filename, ": ", system_msg);
+  if (filename.empty()) filename = "<empty-filename>";
+  const std::string msg = absl::StrCat(filename, ": ", system_msg);
   switch (sys_error) {
     case EPERM:
     case EACCES:
@@ -127,15 +126,17 @@ absl::Status UpwardFileSearch(absl::string_view start,
     if (one_up == probe_dir) break;
     probe_dir = one_up;
   }
-  return absl::NotFoundError("No matching file found.");
+  return absl::NotFoundError(absl::StrCat("UpwardFileSearch: starting from '",
+                                          start, "', no file '", filename,
+                                          "' found'"));
 }
 
 absl::Status FileExists(const std::string &filename) {
   std::error_code err;
   fs::file_status stat = fs::status(filename, err);
 
   if (err.value() != 0) {
-    return absl::NotFoundError(absl::StrCat(filename, ": ", err.message()));
+    return CreateErrorStatusFromErr(filename, err, "file exists check");
   }
 
   if (fs::is_regular_file(stat) || fs::is_fifo(stat)) {