Fix some error message diagnostics

[ablaom]

The diagnostics currently fail in some cases because of access to non-existing fields of a machine when machine (attached to some Node) is actually nothing.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 88.01%. Comparing base (f7ef4fe) to head (542d81f).
Report is 1 commits behind head on dev.

Files	Patch %	Lines
src/composition/learning_networks/nodes.jl	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #962      +/-   ##
==========================================
- Coverage   88.02%   88.01%   -0.02%     
==========================================
  Files          28       28              
  Lines        2548     2554       +6     
==========================================
+ Hits         2243     2248       +5     
- Misses        305      306       +1     

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