better error msg for models without random effects

[palday]

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[codecov]

Codecov Report

Base: 94.23% // Head: 94.06% // Decreases project coverage by -0.17% ⚠️

Coverage data is based on head (bc46d31) compared to base (72ac0e1).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head bc46d31 differs from pull request most recent head cfe54ce. Consider uploading reports for the commit cfe54ce to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #651      +/-   ##
==========================================
- Coverage   94.23%   94.06%   -0.18%     
==========================================
  Files          29       29              
  Lines        2792     2795       +3     
==========================================
- Hits         2631     2629       -2     
- Misses        161      166       +5     

Impacted Files	Coverage Δ
src/linearmixedmodel.jl	98.10% <100.00%> (+<0.01%)	⬆️
src/linalg/pivot.jl	95.00% <0.00%> (-5.00%)	⬇️
src/serialization.jl	90.62% <0.00%> (-3.13%)	⬇️
src/linalg.jl	98.24% <0.00%> (-1.76%)	⬇️
src/generalizedlinearmixedmodel.jl	90.21% <0.00%> (-0.31%)	⬇️
src/remat.jl	96.73% <0.00%> (-0.26%)	⬇️

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