Try and Exception for a specific error from a Linux command output

In case some command we try to execute is not found on the server or a specific error from a command output.

For example:

#!/usr/bin/env python3
import subprocess
os_cmd = 'ech hello'
subprocess.Popen(os_cmd, shell=True)

Error output is:

/bin/sh: 1: ech: not found

How to write exception specific for such command not found error instead of such Built-in Exceptions or instead of just checking only the return code zero or non-zero?

Or is the only way to do it in a dirty way as follows?

#!/usr/bin/env python3

import subprocess

os_cmd = 'ech hello'

p1 = subprocess.Popen(os_cmd, shell=True, stderr=subprocess.PIPE, stdout=subprocess.PIPE)
stdout, stderr = p1.communicate()        
exit_code = p1.poll()
if exit_code != 0:
  string = stderr.decode()
  substring = "not found"
  if substring in string:
    print("It is command not found error")
    print("Do some solution...")
else:
  print("Command succeeded")
  out = stdout.decode().strip().split()[0]
  print(out)

Output:

It is command not found error
Do some solution...