Improve error message for exports for OS2-only targets

[bridadan]

Description

Addresses the issue raised in #8977.

If you try to export an Mbed OS 5 project (includes the rtos) for an OS2-only target, a "not supported" message is supposed to be printed in response to the command. This is currently what you get:

[mbed] Working path "/Users/user/Desktop/test/mbed-os-example-blinky" (program)
Scan: mbed-os-example-blinky
usage: project.py [-h] [-m MCU] [-i IDE] [-c] [-p PROGRAM] [-n PROGRAM] [-b]
                  [-L] [-S [{matrix,ides}]] [--update-packs] [-E]
                  [--build BUILD_DIR] [--source SOURCE_DIR] [-D MACROS]
                  [--profile PROFILE] [--app-config APP_CONFIG] [-z]
                  [--ignore IGNORE]
project.py: error: LPC11U68 not supported by mcuxpresso
[mbed] ERROR: "/Applications/MBEDCLI.app/Contents/Resources/miniconda/bin/python" returned error.
       Code: 2
       Path: "/Users/user/Desktop/test/mbed-os-example-blinky"
       Command: "/Applications/MBEDCLI.app/Contents/Resources/miniconda/bin/python -u /Users/user/Desktop/test/mbed-os-example-blinky/mbed-os/tools/project.py -i mcuxpresso -m LPC11U68 --source ."
       Tip: You could retry the last command with "-v" flag for verbose output

This PR changes the error message to something more relevant:

[mbed] Working path "C:\Users\bridan01\onedrive_arm\Documents\dev\mbed-os-example-blinky" (program)
Scan: mbed-os-example-blinky
[Not Supported] Target does not support mbed OS 5
[mbed] ERROR: "c:\python27\python.exe" returned error.
       Code: 1
       Path: "C:\Users\bridan01\onedrive_arm\Documents\dev\mbed-os-example-blinky"
       Command: "c:\python27\python.exe -u C:\Users\bridan01\onedrive_arm\Documents\dev\mbed-os-example-blinky\mbed-os\tools\project.py -i mcuxpresso -m LPC11U68 --source ."
       Tip: You could retry the last command with "-v" flag for verbose output
---

FYI @fluidblue

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

[naveenkaje]

Looks good to me!

[0xc0170]

Waiting for @theotherjimmy approval, meanwhile running CI

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 12 of 12 test jobs passed
Build number : 1
Build artifacts