layers: WSI functions should not work unless enabled.

The WSI extension functions are special.  They are statically exported from the
loader.  This means that an application can directly link against them, and not
have to call a vkGet*ProcAddr function.  However, the application is still
reponsible for for enabling the appropriate WSI extension.

An application was observed that called the WSI functions without enabling the
appropriate extensions.  It worked.  When that application was run with the
validation layers, it crashed.  A change was made to the layer to make sure
that an error was always logged.  It was decided that the loader should change
so that the terminator functions not work (and print a brief message) if an
application calls the function without enabling its extension.  This is done in
the terminator so that the validation layer can be called, which should log a
good error message; and yet still done here in case the application is never
tried with validation turned on.
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