releasetools: Clean up build_image.py.

Mostly cosmetic changes, such as moving print statement to print
function. The only functional change is to wrap the file opening in
Append() with try statement, which would dump the error message as
needed.

Test: `m dist`
Test: python -m unittest test_build_image
Change-Id: I4e6c593517cf737bfcbe51bc533b22a247b10e44