Fail `update-tags` if not tot revision is found (#574)

diff --git a/emsdk.py b/emsdk.py
index f3ab07d..2f4393d 100755
--- a/emsdk.py
+++ b/emsdk.py
@@ -2051,7 +2051,7 @@
     except:
       continue
     return release
-  return ''
+  exit_with_error('failed to find build of any recent emsdk revision')
 
 
 def get_release_hash(arg, releases_info):
@@ -2092,8 +2092,7 @@
   if git:
     print('Fetching emscripten-releases repository...')
     emscripten_releases_tot = get_emscripten_releases_tot()
-    if emscripten_releases_tot:
-      open(tot_path(), 'w').write(emscripten_releases_tot)
+    open(tot_path(), 'w').write(emscripten_releases_tot)
   else:
     print('Update complete, however skipped fetching the Emscripten tags, since git was not found, which is necessary for update-tags.')
     if WINDOWS: