Improve error message in npm_package if a file fails to copy
Improve error message in npm_package if a file fails to copy

bazelbuild/rules_nodejs,bazelbuild/rules_nodejs,bazelbuild/rules_nodejs,alexeagle/rules_nodejs,alexeagle/rules_nodejs,bazelbuild/rules_nodejs,alexeagle/rules_nodejs,bazelbuild/rules_nodejs,alexeagle/rules_nodejs