epp: Add the -error and -warning directives

If one of several alternatives configurations are required for
an Erlang module to compile, but none are available, it would
be useful to give a nice error message. For example:

  -ifdef(CONFIG_A).
     %% Some code if A is true.
  -else.
  -ifdef(CONFIG_B).
    %% Some code if B is true.
  -else.
    -error("Neither CONFIG_A nor CONFIG_B are available").
  -endif.
  -endif.

If neither CONFIG_A nor CONFIG_B are defined, the error message
will be:

  module.erl:10: -error("Neither CONFIG_A nor CONFIG_B are available").

That is basically the same behavior as for the #error directive in
GCC.

For symmetry with the -error directive, add the -warning
directive to generate a compiler warning. For example:

  -ifdef(HAVE_COOL_FEATURE).
     %% Code if we have Cool Feature.
  -else.
     %% Inefficient fallback code.
     -warning("Using inefficient fallback").
  -endif.

If HAVE_COOL_FEATURE is not defined, the warning message will
be:

  module.erl:8: Warning: -warning("Using inefficient fallback").

That is basically the same behavior as for the #warning directive
in GCC.

Conflicts:
	lib/stdlib/src/epp.erl
	lib/stdlib/test/epp_SUITE.erl

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