[Mellanox] fix sfp issue in reading qsfp_status (#3386)

There is a very low possibility that qsfp_status exists when opening it but doesn't when reading it a bit later. If this occurs an exception is raised.
However, this should be treated as "not presence" rather than causing an exception, which can be achieved by putting it into try block.

device/mellanox/x86_64-mlnx_msn2700-r0/plugins/sfputil.py

@@ -8,6 +8,7 @@ try:
     import subprocess
     from sonic_sfp.sfputilbase import *
     import syslog
+    import sys
 except ImportError as e:
     raise ImportError("%s - required module not found" % str(e))
 
@@ -103,11 +104,10 @@ class SfpUtil(SfpUtilBase):
 
         try:
             reg_file = open(self.qsfp_sysfs_path + "qsfp{}_status".format(port_num + 1))
-        except IOError as e:
-            print "Error: unable to open file: %s" % str(e)
-            return False
-
             content = reg_file.readline().rstrip()
+        except IOError as e:
+            print >> sys.stderr, "Error: unable to access file: %s" % str(e)
+            return False
 
         # content is a string with the qsfp status
         if content == SFP_STATUS_INSERTED:
@@ -127,7 +127,7 @@ class SfpUtil(SfpUtilBase):
             if 'LPM ON' in output:
                 return True
         except subprocess.CalledProcessError as e:
-            print "Error! Unable to get LPM for {}, rc = {}, err msg: {}".format(port_num, e.returncode, e.output)
+            print >> sys.stderr, "Error! Unable to get LPM for {}, rc = {}, err msg: {}".format(port_num, e.returncode, e.output)
             return False
 
         return False