Duplicate implicit target names cause ugly error messages

[mgedmin]

See https://stackoverflow.com/questions/39717529/duplicate-implicit-target-name-how-to-use-a-subtitle-name-more-than-once: text like

Document title
==============

Section 1
---------

Description
~~~~~~~~~~~

Section 2
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Description
~~~~~~~~~~~

causes (INFO/1) Duplicate implicit target name: "description" under Section 2. Adding explicit target names (via .. _sec2_description: above the heading) doesn't help -- instead it adds a new error at the bottom, about an unused target.

We should hide these errors by default. We should probably hide all INFO messages by default.