Consistently check matrix sizes in matmul

[jishnub]

Fixes #1147, and makes the error messages more verbose.

Before

julia> zeros(0,4) * zeros(1)
ERROR: DimensionMismatch: second dimension of matrix, 4, does not match length of input vector, 1
[...]

julia> mul!(zeros(0), zeros(2,2), zeros(2))
ERROR: DimensionMismatch: first dimension of matrix, 2, does not match length of output vector, 0
[...]

After

julia> zeros(0,4) * zeros(1)
ERROR: DimensionMismatch: incompatible dimensions for matrix multiplication: tried to multiply a matrix of size (0, 4) with a vector of length 1. The second dimension of the matrix: 4, does not match the length of the vector: 1.
[...]

julia> zeros(0,4) * zeros(1,1)
ERROR: DimensionMismatch: incompatible dimensions for matrix multiplication: tried to multiply a matrix of size (0, 4) with a matrix of size (1, 1). The second dimension of the first matrix: 4, does not match the first dimension of the second matrix: 1.
[...]

julia> mul!(zeros(0), zeros(2,2), zeros(2))
ERROR: DimensionMismatch: incompatible destination size: the destination vector of length 0 is incomatible with the product of a matrix of size (2, 2) and a vector of length 2. The destination must be of length 2.
[...]

julia> mul!(zeros(0,1), zeros(3,2), zeros(2,3))
ERROR: DimensionMismatch: incompatible destination size: the destination matrix of size (0, 1) is incomatible with the product of a matrix of size (3, 2) and a matrix of size (2, 3). The destination must be of size (3, 3).
[...]

[dkarrasch]

Beautiful! Are these all size checks in matmul? I'd expect it in every matrix type file, like triangular etc.

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 4 lines in your changes missing coverage. Please review.

Project coverage is 91.87%. Comparing base (6e5ea12) to head (9f1edba).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/matmul.jl	93.33%	3 Missing ⚠️
src/triangular.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1152      +/-   ##
==========================================
+ Coverage   91.83%   91.87%   +0.04%     
==========================================
  Files          34       34              
  Lines       15363    15355       -8     
==========================================
  Hits        14108    14108              
+ Misses       1255     1247       -8     

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