Better init_theta size mismatch error

[alyst]

Just a tiny tweak. Providing how exactly the sizes do not match helps debugging.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.47%. Comparing base (a020171) to head (63c0354).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #576   +/-   ##
=======================================
  Coverage   84.47%   84.47%           
=======================================
  Files          28       28           
  Lines        3207     3207           
=======================================
  Hits         2709     2709           
  Misses        498      498           

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[coveralls]

Pull Request Test Coverage Report for Build 8102638906

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 25 unchanged lines in 4 files lost coverage.
• Overall coverage remained the same at 84.789%

---

Files with Coverage Reduction	New Missed Lines	%
src/model.jl	1	89.22%
src/simple_varinfo.jl	6	80.66%
src/varinfo.jl	6	92.6%
src/threadsafe.jl	12	53.1%

Totals
Change from base Build 8101420428:	0.0%
Covered Lines:	2709
Relevant Lines:	3195

---

💛 - Coveralls

[yebai]

Thanks @alyst!