Improve warning/error formatting and various messages

[screamerbg]

This PR introduces cosmetic changes to Mbed CLI to improve error and warning multi-line formatting and also the wording of some messages.

Old multi-line formatting

$ mbed compile -m DETECT
[mbed] Auto-installing missing Python modules...
[mbed] WARNING: Unable to auto-install required Python modules.
---
[mbed] WARNING: -----------------------------------------------------------------
[mbed] WARNING: The mbed OS tools in this program require the following Python modules: intelhex
[mbed] WARNING: You can install all missing modules by running "pip install -r requirements.txt" in "/private/tmp/1/mbed-os-example-blinky/mbed-os"
[mbed] WARNING: On Posix systems (Linux, Mac, etc) you might have to switch to superuser account or use "sudo"
---
[mbed] Detected "DISCO_L475VG_IOT01A" connected to "/Volumes/DIS_L4IOT" and using com port "/dev/tty.usbmodem1413"
Traceback (most recent call last):
  File "/private/tmp/1/mbed-os-example-blinky/mbed-os/tools/make.py", line 34, in <module>
    from tools.utils import args_error
  File "/private/tmp/1/mbed-os-example-blinky/mbed-os/tools/utils.py", line 32, in <module>
    from intelhex import IntelHex
ImportError: No module named intelhex
[mbed] ERROR: "/System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python" returned error code 1.
[mbed] ERROR: Command "/System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python -u /private/tmp/1/mbed-os-example-blinky/mbed-os/tools/make.py -t GCC_ARM -m DISCO_L475VG_IOT01A --source . --build ./BUILD/DISCO_L475VG_IOT01A/GCC_ARM" in "/private/tmp/1/mbed-os-example-blinky"
---

New multi-line formatting

$ mbed compile -m DETECT
[mbed] Auto-installing missing Python modules...
[mbed] WARNING: Unable to auto-install required Python modules.
       The mbed OS tools in this program require the following Python modules: intelhex
       You can install all missing modules by running "pip install -r requirements.txt" in "/private/tmp/1/mbed-os-example-blinky/mbed-os"
       On Posix systems (Linux, Mac, etc) you might have to switch to superuser account or use "sudo"
---
[mbed] Detected "DISCO_L475VG_IOT01A" connected to "/Volumes/DIS_L4IOT" and using com port "/dev/tty.usbmodem1413"
Traceback (most recent call last):
  File "/private/tmp/1/mbed-os-example-blinky/mbed-os/tools/make.py", line 34, in <module>
    from tools.utils import args_error
  File "/private/tmp/1/mbed-os-example-blinky/mbed-os/tools/utils.py", line 32, in <module>
    from intelhex import IntelHex
ImportError: No module named intelhex
[mbed] ERROR: "/System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python" returned error.
       Code: 1
       Path: "/private/tmp/1/mbed-os-example-blinky"
       Command: "/System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python -u /private/tmp/1/mbed-os-example-blinky/mbed-os/tools/make.py -t GCC_ARM -m DISCO_L475VG_IOT01A --source . --build ./BUILD/DISCO_L475VG_IOT01A/GCC_ARM"
       Tip: You could retry the last command with "-v" flag for verbose output
---

Related to #683 (comment)

CC @ansondtx20

[theotherjimmy]

Looks fine to me. You should look at the Python 3 CI failures.

[screamerbg]

@theotherjimmy Now fixed.
Bump @MarceloSalazar

[cmonr]

LGTM, just a question about a new exception.

[cmonr]

Interesting. What command is used to invoke this error?

[MarceloSalazar]

LGTM