Avoid runtime exception when file doesn't exist (#2441)

* Avoid runtime exception when file doesn't exist

* Update the check based on feedback

* Revert the old fix

python/tvm/autotvm/tophub.py

@@ -77,7 +77,8 @@ def context(target, extra_files=None):
         for name in possible_names:
             name = _alias(name)
             if name in all_packages:
-                check_backend(name)
+                if not check_backend(name):
+                    continue
 
                 filename = "%s_%s.log" % (name, PACKAGE_VERSION[name])
                 best_context.load(os.path.join(AUTOTVM_TOPHUB_ROOT_PATH, filename))
@@ -98,23 +99,30 @@ def check_backend(backend):
     ----------
     backend: str
         The name of backend.
+
+    Returns
+    ----------
+    success: bool
+        Whether the check is successful.
     """
     backend = _alias(backend)
     assert backend in PACKAGE_VERSION, 'Cannot find backend "%s" in TopHub' % backend
 
     version = PACKAGE_VERSION[backend]
     package_name = "%s_%s.log" % (backend, version)
     if os.path.isfile(os.path.join(AUTOTVM_TOPHUB_ROOT_PATH, package_name)):
-        return
+        return True
 
     if sys.version_info >= (3,):
         import urllib.request as urllib2
     else:
         import urllib2
     try:
         download_package(package_name)
+        return True
     except urllib2.URLError as e:
         logging.warning("Failed to download tophub package for %s: %s", backend, e)
+        return False
 
 
 def download_package(package_name):