BaseTools: remove unnecessary calls of os.exist

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2101

This patch is going to remove unnecessary calls
of os.exist()

Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/Python/AutoGen/GenMake.py

@@ -636,11 +636,13 @@ cleanlib:
         while not found and os.sep in package_rel_dir:
             index = package_rel_dir.index(os.sep)
             current_dir = mws.join(current_dir, package_rel_dir[:index])
-            if os.path.exists(current_dir):
+            try:
                 for fl in os.listdir(current_dir):
                     if fl.endswith('.dec'):
                         found = True
                         break
+            except:
+                EdkLogger.error('build', FILE_NOT_FOUND, "WORKSPACE does not exist.")
             package_rel_dir = package_rel_dir[index + 1:]
 
         MakefileTemplateDict = {