[RTR] empty initial guess / scaling problem

[EveCharbie]

Modified the error message in response to issue #622.

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This change is 

[codecov]

Codecov Report

Patch coverage: 90.00% and project coverage change: +0.45 🎉

Comparison is base (5ed2dbd) 81.11% compared to head (adaf73e) 81.57%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #666      +/-   ##
==========================================
+ Coverage   81.11%   81.57%   +0.45%     
==========================================
  Files         112      112              
  Lines       12906    12949      +43     
==========================================
+ Hits        10469    10563      +94     
+ Misses       2437     2386      -51     

Impacted Files	Coverage Δ
...ptim/optimization/receding_horizon_optimization.py	94.20% <75.00%> (-0.90%)	⬇️
bioptim/optimization/optimal_control_program.py	86.80% <100.00%> (+0.22%)	⬆️
bioptim/optimization/optimization_variable.py	87.54% <100.00%> (+6.83%)	⬆️

... and 34 files with indirect coverage changes

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[pariterre]

Reviewed 1 of 2 files at r1, 1 of 2 files at r2, 1 of 2 files at r3, 2 of 2 files at r4, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @EveCharbie)