world: hide unknown block/biome type warnings by default

While using MinedMap with modded Minecraft version is not officially
supported, it should still work reasonably well if you're okay with
custom block types being invisible and custom biomes using default
grass/color/foliage colors.

Avoid spamming the log with messages for each section in this case
without --verbose, and instead just print a single warning at the end of
processing.

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 ## [Unreleased] - ReleaseDate
 
+### Changed
+
+- Without `--verbose`, only a single warning is printed at the end of
+  processing for unknown block/biome types, rather than once for every
+  section where such a block/biome is encountered.
+
 ## [2.0.2] - 2024-01-07
 
 ### Added