This commit adds human readable error messages when mbedtls or axtls raise
an exception.  Currently often just an EIO error is raised so the user is
lost and can't tell whether it's a cert error, buffer overrun, connecting
to a non-ssl port, etc.  The axtls and mbedtls error raising in the ussl
module is modified to raise:

    OSError(-err_num, "error string")

For axtls a small error table of strings is added and used for the second
argument of the OSErrer.  For mbedtls the code uses mbedtls' built-in
strerror function, and if there is an out of memory condition it just
produces OSError(-err_num).  Producing the error string for mbedtls is
conditional on them being included in the mbedtls build, via
MBEDTLS_ERROR_C.

The file `mbedtls_errors/mp_mbedtls_errors.c` can be used instead of
`mbedtls/library/error.c` to give shorter error strings, reducing the build
size of the error strings from about 12-16kB down to about 2-5kB.

The stm32 and esp32 ports now use shorter error strings for mbedtls errors.
Also, MBEDTLS_ERROR_C is enabled on stm32 by default to get these strings.