Fix cprintf bug, plus minor cleanup & better error messages.

Fix cprintf bug, plus minor cleanup & better error messages.

base/cprintf.cc:
    Fix bug where a call with a format string with no specifiers
    but one or more arguments would crash.  Old code tried to print
    extra args using last available format, but in this case there
    was no format, hence the problem.  Now we just print "<extra arg>"
    for each extra arg.

    Also reorganized code a bit to make scope of fmt variable
    match the scope in which you can be confident it's meaningful.
base/cprintf.hh:
    Print specific error message instead of calling format_invalid().
base/cprintf_formats.hh:
    Clear Format object in constructor.
    Use specific error messages instead of format_invalid() function.

--HG--
extra : convert_revision : 87d5e902174e3eb2583131d056742af166540db0