Fix nbtree page deletion error messages.

Adjust some "can't happen" error messages that assumed that the page
deletion target page must be a half-dead page.  This assumption was
wrong in the case of an internal target page.  Simply refer to these
pages as the target page instead.

Internal pages are never marked half-dead.  There is exactly one
half-dead page for each subtree undergoing deletion.  The half-dead page
is also the target subtree's leaf-level page.  This has been the case
since commit efada2b8, which totally overhauled nbtree page deletion.

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