Fix error message formatting in uf2conv

[PetteriAimonen]

Some of the error messages were trying to concatenate int to str.
This gave error such as:

Traceback (most recent call last):
  File "uf2conv.py", line 363, in <module>
    main()
    ~~~~^^
  File "uf2conv.py", line 327, in main
    outbuf = convert_from_uf2(inpbuf)
  File "uf2conv.py", line 69, in convert_from_uf2
    assert False, "Block out of order at " + ptr
                  ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
TypeError: can only concatenate str (not "int") to str

Fixed the formatting and made the errors more helpful by including the addresses involved.

Converted the if foo: assert False construct to more idiomatic assert !foo. There is no performance penalty as Python does not evaluate the second argument of assert unless it fails.