verific: New -sva-continue-on-error import option

This option allows you to process a design that includes unsupported
SVA. Unsupported SVA gets imported as formal cells using 'x inputs and
with the `unsupported_sva` attribute set. This allows you to get a
complete list of defined properties or to check only a supported subset
of properties. To ensure no properties are unintentionally skipped for
actual verification, even in cases where `-sva-continue-on-error` is
used by default to read and inspect a design, `hierarchy -simcheck` and
`hierarchy -smtcheck` (run by SBY) now ensure that no `unsupported_sva`
property cells remain in the design.

tests/verific/sva_no_continue_on_err.ys

@@ -0,0 +1,9 @@
+
+verific -sv <<EOF
+module top(input clk, input a, input b);
+    prop_unsupported: assert property (@(posedge clk) a ##1 @(posedge b) ##1 a);
+endmodule;
+EOF
+
+logger -expect error "Mixed clocking is currently not supported" 1
+verific -import top