Annotate ColoredPrintf with the format attribute and fix bugs.

googletest doesn't currently build with clang's very aggressive
-Wformat-nonliteral warning. It requires that all non-literal format
strings come from the argument of a function annotated with a compatible
format attribute.

Fixing that reports that ColoredPrintf's callers weren't passing the
normal -Wformat warning. Some messages were passed directly into the
format string rather than via "%s".