Check Errorf()/ErrnoErrorf() format string at compile time

fmtlib provides compile time checking of format strings that we're not
currently using.  This change makes Errorf() and ErrnoErrorf() into
macros such that we can take advantage of this capability.

Test: build successfully normally
Test: fail the build if using an invalid format string
Change-Id: Icb8ba8cb973bbd1fa4755a62e7598bdbb0113757