change to make - error when Makefile doesn't exist

[Eitan Adler]

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I stupidly forgot to make a backup so I can't provide a diff but in
src/usr.bin
I changed
         TryReadMakefile("Makefile");
to
          if (!TryReadMakefile("Makefile"))
                  Fatal("Makefile could not be opened");
That way you could differentiate between the Makefile not existing and
the target not existing.

gmake provided the following error message:
gmake: *** No targets specified and no makefile found.  Stop.

- --
Eitan Adler
GNU Key fingerptrint: 2E13 BC16 5F54 0FBD 62ED 42B6 B65F 24AB E9C2 CCD1
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