Handle syntax errors in `mpy-cross`, specially during watch

[gprossliner]

When on watch command, and a syntax error has been introduced into any source file, mpy-cross exists with an error code, causing the program to quit:

For building this is not a big issue, as the error needs to be fixed anyway, and we can't continue.

For watch is not ideal, since that ends the sesison, and you need to restart it. I think it should be handled in a better way. We present the error message, bc it will show what the user need to fix, as seen by the attached screenshot.