Reduce error to debug and add notes

[mjcarroll]

Partially addresses #450 by reducing the verbosity of the two errors from "error" to "debug".

In reality, these aren't really errors per se, as the dartsim plugin continues to process the SDF and then attaches meshes and heightmaps in a later stage.

The recent addition of gazebosim/gz-sim#1560 allowing SDF to be constructed in a single shot uncovered these unimplemented functions.

The more complete fix is to take advantage of constructing the models in place, which I have opening an issue for at #451.

[codecov]

Codecov Report

Merging #452 (f4111cb) into gz-physics6 (bab13ef) will not change coverage.
The diff coverage is 0.00%.

❗ Current head f4111cb differs from pull request most recent head 077d46b. Consider uploading reports for the commit 077d46b to get more accurate results

@@             Coverage Diff              @@
##           gz-physics6     #452   +/-   ##
============================================
  Coverage        75.79%   75.79%           
============================================
  Files              140      140           
  Lines             7056     7056           
============================================
  Hits              5348     5348           
  Misses            1708     1708           

Impacted Files	Coverage Δ
dartsim/src/SDFFeatures.cc	64.07% <0.00%> (ø)

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