noncliff: improve error handling in apply!(sm, pcT)

[Fe-r-oz]

This PR aims to improve the error handling for apply!(sm, pcT).

Before:

julia> p = embed(2, 1, pcT)
A unitary Pauli channel P = ∑ ϕᵢ Pᵢ with the following branches:
with ϕᵢ | Pᵢ
 0.853553+0.353553im | + __
 0.146447-0.353553im | + Z_

julia> sm = GeneralizedStabilizer(S"X")
A mixture ∑ ϕᵢⱼ Pᵢ ρ Pⱼ† where ρ is
𝒟ℯ𝓈𝓉𝒶𝒷
+ Z
𝒮𝓉𝒶𝒷
+ X
with ϕᵢⱼ | Pᵢ | Pⱼ:
 1.0+0.0im | + _ | + _

julia> apply!(sm, p)
ERROR: BoundsError: attempt to access 2×1 view(::Matrix{UInt64}, :, 2:2) with eltype UInt64 at index [1:2, 2]
Stacktrace:
  [1] throw_boundserror(A::SubArray{UInt64, 2, Matrix{…}, Tuple{…}, true}, I::Tuple{Base.Slice{…}, Int64})
    @ Base ./abstractarray.jl:737
  [2] checkbounds
    @ ./abstractarray.jl:702 [inlined]
  [3] view
    @ ./subarray.jl:184 [inlined]
...
Some type information was truncated. Use `show(err)` to see complete types.

After:

julia> sm = GeneralizedStabilizer(random_stabilizer(2))
A mixture ∑ ϕᵢⱼ Pᵢ ρ Pⱼ† where ρ is
𝒟ℯ𝓈𝓉𝒶𝒷
+ Z_
+ _Z
𝒮𝓉𝒶𝒷
+ YZ
+ ZX
with ϕᵢⱼ | Pᵢ | Pⱼ:
 1.0+0.0im | + __ | + __

julia> apply!(sm, pcT)
ERROR: DimensionMismatch: GeneralizedStabilizer has 2 qubits, but PauliChannel has 1. Use `embed` to create an appropriately padded PauliChannel.
Stacktrace:
 [1] apply!(state::GeneralizedStabilizer{…}, gate::PauliChannel{…}; prune_threshold::Float64)
...

• The code is properly formatted and commented.
• Substantial new functionality is documented within the docs.
• All new functionality is tested.
• All of the automated tests on github pass.

[Fe-r-oz]

The PR is ready for review. Thank you!

[Krastanov]

thank you!