More helpful error message when main.min.js can't be found.

[tkoolen]

Fixes #63.

[codecov-io]

Codecov Report

Merging #64 into master will not change coverage.
The diff coverage is 0%.

@@         Coverage Diff          @@
##           master   #64   +/-   ##
====================================
  Coverage       0%    0%           
====================================
  Files          12    12           
  Lines         301   304    +3     
====================================
- Misses        301   304    +3

Impacted Files	Coverage Δ
src/MeshCat.jl	0% <0%> (ø)	⬆️

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[rdeits]

thanks!