Bugfix subprocess error handling

[PhilippeMoussalli]

Setting capture_output=True captures both the default outputstdout and the error output stderr in the output variable which means that we're not streaming anymore the standard output of docker-compose up.

This PR fixes this by only capturing the error stderr in the output, the standard output is streamed

Before:

Traceback (most recent call last):
  File "/home/philippe/Scripts/express/a/launch.py", line 14, in <module>
    raise Exception(msg)
Exception: Command failed with error: ' Container a-web-1  Created
a-web-1  | Traceback (most recent call last):
a-web-1  |   File "app.py", line 4, in <module>
a-web-1  |     print(1 / 0)
a-web-1  | ZeroDivisionError: division by zero
Aborting on container exit...
 Container a-web-1  Stopping
 Container a-web-1  Stopped

After:

Attaching to a-web-1
a-web-1  | This is a simple Python script.  #Standard output of print statement contained in the script 
a-web-1 exited with code 1
Traceback (most recent call last):
  File "/home/philippe/Scripts/express/a/launch.py", line 14, in <module>
    raise Exception(msg)
Exception: Command failed with error: ' Container a-web-1  Created
a-web-1  | Traceback (most recent call last):
a-web-1  |   File "app.py", line 4, in <module>
a-web-1  |     print(1 / 0)
a-web-1  | ZeroDivisionError: division by zero
Aborting on container exit...
 Container a-web-1  Stopping
 Container a-web-1  Stopped

[RobbeSneyders]

Is it an issue that stderr is no longer streamed? I think there's ways around it, but they require using Popen directly I believe.

[PhilippeMoussalli]

Is it an issue that stderr is no longer streamed? I think there's ways around it, but they require using Popen directly I believe.

Not streaming stderr is fine because we will raise it anyways when we catch the error. I think the issue is mainly with streaming stdout since we don't have that logs that are appearing anymore. Setting capture_output=True seem to block streaming for both output types which is why now we only capture stderr

[PhilippeMoussalli]

Closed in favor of #775