findfirst error message on UTF8Strings confusing

[jiahao]

julia> findfirst("你好", "不好")
ERROR: UnicodeError: invalid character index
 in next(::String, ::Int64) at ./unicode/utf8.jl:65
 in getindex at ./strings/basic.jl:58 [inlined]
 in findnext(::String, ::String, ::Int64) at ./array.jl:745
 in findfirst(::String, ::String) at ./array.jl:751
 in eval(::Module, ::Any) at ./boot.jl:225
 in macro expansion at ./REPL.jl:92 [inlined]
 in (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at ./event.jl:46

I realize this is not supposed to work, but a better error message is useful.

Reproducible on master and release-0.4.

[nalimilan]

This could perfectly be made to work. Since #15755 won't happen in 0.5, we could write specific methods for strings (which are the same as the ones for general iterables in that PR, except that we know we can return 0 for not found).

[KristofferC]

Close as dup of #15723.