BaseTools: Refactor python except statements

Convert "except ... ," to "except ... as" to be compatible with python3.
Based on "futurize -f lib2to3.fixes.fix_except"

Contributed-under: TianoCore Contribution Agreement 1.1
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>

BaseTools/Source/Python/GenFds/GenFdsGlobalVariable.py

@@ -721,7 +721,7 @@ class GenFdsGlobalVariable:
 
         try:
             PopenObject = subprocess.Popen(' '.join(cmd), stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)
-        except Exception, X:
+        except Exception as X:
             EdkLogger.error("GenFds", COMMAND_FAILURE, ExtraData="%s: %s" % (str(X), cmd[0]))
         (out, error) = PopenObject.communicate()