warn early about incorrect `sysimage_path`

[sjkelly]

This possible error happens late in the build, so this change will make it error earlier for the user. Should improve experience a bit.

[codecov[bot]]

Codecov Report

Merging #822 (a2998ec) into master (dff7b2c) will decrease coverage by 1.47%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #822      +/-   ##
==========================================
- Coverage   82.59%   81.12%   -1.47%     
==========================================
  Files           3        3              
  Lines         655      657       +2     
==========================================
- Hits          541      533       -8     
- Misses        114      124      +10     

Impacted Files	Coverage Δ
src/PackageCompiler.jl	91.08% <50.00%> (-1.72%)	:arrow_down:

... and 1 file with indirect coverage changes