fix error messages from set methods

src/tests/set.vtc

@@ -123,7 +123,7 @@ client c1 {
 	expect resp.http.empty_anchored2 == <undef>
 } -run
 
-varnish v1 -errvcl {vmod re2 error: unanchored.add("("): missing ): (} {
+varnish v1 -errvcl {vmod re2 error: unanchored.add("("): Cannot compile '(': missing ): (} {
 	import re2 from "${vmod_topbuild}/src/.libs/libvmod_re2.so";
 	backend b { .host = "${bad_ip}"; }
 
@@ -136,7 +136,7 @@ varnish v1 -errvcl {vmod re2 error: unanchored.add("("): missing ): (} {
 	}
 }
 
-varnish v1 -errvcl {vmod re2 error: anchored.add("("): missing ): (} {
+varnish v1 -errvcl {vmod re2 error: anchored.add("("): Cannot compile '(': missing ): (} {
 	import re2 from "${vmod_topbuild}/src/.libs/libvmod_re2.so";
 	backend b { .host = "${bad_ip}"; }
 

src/vmod_re2.c

@@ -446,14 +446,15 @@ vmod_set_add(VRT_CTX, struct vmod_re2_set *set, VCL_STRING pattern)
 
 	/* XXX: check if compiled */
 	if ((err = vre2set_add(set->set, pattern)) != NULL) {
-		VERR(ctx, ERR_PREFIX "%s", set->vcl_name, pattern, err);
+		VERR(ctx, ERR_PREFIX "Cannot compile '%s': %s", set->vcl_name,
+		     pattern, pattern, err);
 		return;
 	}
 }
 
 #undef ERR_PREFIX
 
-#define ERR_PREFIX "%s.compile(\"%s\"): "
+#define ERR_PREFIX "%s.compile(): "
 
 VCL_VOID
 vmod_set_compile(VRT_CTX, struct vmod_re2_set *set)