Fail gracefully when shell configuration is not accessible

[mkitti]

#827 shows that juliaup will fail if it cannot modify the user's shell configuration. Is there a clearer way to recommend to the user that the issue may be file permissions on that shell configuration? Otherwise, is there a way for installation to succeed while recommending that the user manually insert lines into a shell configuration or otherwise make Julia available on the user's PATH.

ref: JuliaLang/Pkg.jl#3772 (comment)