This PR adds the errno attribute to exceptions, so code can retrieve errno codes from an OSError using exc.errno.

This is an alternative implementation of #2407, which generated a lot of controversy at the time (4.5 years ago).

The reason to post this PR is to hopefully finish the discussion about adding/not adding this feature, and finally close #2407.

The implementation here is a simpler version of #2407 which simply lets errno (and the existing value) attributes to work on any exception instance (they both alias args[0]). This is for efficiency and to keep code size down. As I see it, the pros and cons of adding errno are:

Pros:

• more compatible with CPython, less difference to document and learn
• OSError().errno will correctly return None, whereas the current way of doing it via OSError().args[0] will raise an IndexError
• actually reduces code size on most bare-metal ports (because they already have the errno qstr)
• for Python code that uses exc.errno the generated bytecode is 2 bytes smaller and more efficient to execute (compared with exc.args[0]); so bytecode loaded to RAM saves 2 bytes RAM for each use of this attribute, and bytecode that is frozen saves 2 bytes flash/ROM for each use
• it's easier/shorter to type, and saves 2 bytes of space in .py files that use it (for each use)

Cons:

• increases code size by 4-8 bytes on minimal ports that don't enable the uerrno module (due to the additional qstr)
• all exceptions now have .errno and .value attributes (this is specific to the implementation here)

Feedback/comments welcome!

Code size change of this PR:

   bare-arm:    +4 +0.006% 
minimal x86:    +8 +0.005% 
   unix x64:    +0 +0.000% 
unix nanbox:   +20 +0.004% 
      stm32:   -12 -0.003% PYBV10
     cc3200:   -16 -0.009% 
    esp8266:    -4 -0.001% GENERIC
      esp32:    -8 -0.001% GENERIC
        nrf:    +0 +0.000% pca10040
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
        rp2:   -24 -0.005% PICO