RFR: Use properly-scoped FormatBuffers instead of err_msg when message is retained

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/formatbuffers/webrev.01/

It seems we cannot use err_msg the way we do it now, when its result is retained for later. In this
case, we have to use the properly-scoped FormatBuffer that has clear lifetime. Otherwise, on some
platforms and compilers we have this:

[0.620s][info][gc] GC(0) Pause Init Mark 5.032ms
[0.661s][info][gc] GC(0) \u0008 1M->1M(7912M) 40.084ms
[0.708s][info][gc] GC(0) \u0008 47.164ms

Testing: hotspot_gc_shenandoah, eyeballing the GC logs on failing configs

Thanks,
-Aleksey