[ssd] Allow individual vendor parsers to handle errors (#252)

Description
Removed a return None in the command execution helper function _execute_shell in the SSD class so that command output is still parsed in the event of a non-zero exit code.

Motivation and Context
If there is an issue found with a SSD SMART utilities may return a non-zero error code and we would still want to parse the output so we should not bail.

In addition, with the return None the parsing functions throw stack traces during execution. When removed, the parsing functions are able to elegantly handle missing information and the fields are left as N/A so there is no need for handling here.

How Has This Been Tested?
Tested manually on a SN2010 with a malfunctioning SSD which was initially throwing stack traces and now handles the error elegantly reporting the health as N/A a much more informative error than a python stack trace.

sonic_platform_base/sonic_ssd/ssd_generic.py

@@ -63,9 +63,6 @@ def __init__(self, diskdev):
     def _execute_shell(self, cmd):
         process = subprocess.Popen(cmd.split(), universal_newlines=True, stdout=subprocess.PIPE)
         output, error = process.communicate()
-        exit_code = process.returncode
-        if exit_code:
-            return None
         return output
 
     def _parse_re(self, pattern, buffer):