Ignoring pcre_study() error.

It provides optional optimization that shouldn't be fatal for regex compilation.
diff --git a/src/njs_pcre.c b/src/njs_pcre.c
index c8fa5ba..2fafea7 100644
--- a/src/njs_pcre.c
+++ b/src/njs_pcre.c
@@ -94,10 +94,8 @@
     regex->extra = pcre_study(regex->code, 0, &errstr);
 
     if (njs_slow_path(errstr != NULL)) {
-        njs_alert(ctx->trace, NJS_LEVEL_ERROR,
+        njs_alert(ctx->trace, NJS_LEVEL_WARN,
                   "pcre_study(\"%s\") failed: %s", pattern, errstr);
-
-        goto done;
     }
 
     err = pcre_fullinfo(regex->code, NULL, PCRE_INFO_CAPTURECOUNT,