Improve error message for initial_params

[sunxd3]

Address #774 partially.

This only improve the error message, and does not add support for using OrderedDIct for initializing.

The error messages are:

• for Vector{Any}

ArgumentError: `initial_params` must be a vector of type `Union{Real,Missing}`. If `initial_params` is a vector of vectors, please flatten it first using `vcat`.

• for NamedTuple (keys of NamedTuple subsumes some variables in the model, but itself not in the model)

ArgumentError: Variable X not found in model, but it subsumes a variable (X[1]) in the model. Please use AbstractVector for initial_params instead of NamedTuple.

[yebai]

Cc @willtebbutt, who has a lot of experience with good error messages and robust user interfaces.

[codecov]

Codecov Report

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

Project coverage is 86.13%. Comparing base (3d18cfc) to head (d267fc6).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/sampler.jl	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #772      +/-   ##
==========================================
+ Coverage   86.12%   86.13%   +0.01%     
==========================================
  Files          36       36              
  Lines        4324     4336      +12     
==========================================
+ Hits         3724     3735      +11     
- Misses        600      601       +1     

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[coveralls]

Pull Request Test Coverage Report for Build 12650555390

Details

• 0 of 11 (0.0%) changed or added relevant lines in 1 file are covered.
• 1190 unchanged lines in 25 files lost coverage.
• Overall coverage decreased (-29.1%) to 57.149%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/sampler.jl	0	11	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/DynamicPPL.jl	1	90.91%
src/submodel_macro.jl	2	82.35%
src/selector.jl	3	42.86%
src/test_utils/varinfo.jl	5	78.26%
src/test_utils/model_interface.jl	5	22.22%
src/distribution_wrappers.jl	6	34.29%
src/logdensityfunction.jl	6	61.76%
src/model_utils.jl	7	7.14%
src/model.jl	12	80.87%
src/test_utils/contexts.jl	12	58.14%

Totals
Change from base Build 12602182198:	-29.1%
Covered Lines:	2470
Relevant Lines:	4322

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 12656520821

Details

• 11 of 12 (91.67%) changed or added relevant lines in 1 file are covered.
• 29 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.02%) to 86.219%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/sampler.jl	11	12	91.67%

Files with Coverage Reduction	New Missed Lines	%
src/varinfo.jl	3	84.92%
src/model.jl	5	77.39%
src/threadsafe.jl	21	46.61%

Totals
Change from base Build 12602182198:	0.02%
Covered Lines:	3735
Relevant Lines:	4332

---

💛 - Coveralls

[willtebbutt]

LGTM

[penelopeysm]

Thanks for this PR! It's great that people will get a clearer error message especially on this awkward bit with subsumed varnames :)

Just suggesting some very minor changes:

[penelopeysm]

feel free to add to merge queue 😄