Fix potential instability/invalidation in dlpath (#53232)
While debugging some invalidations and instabilities in code in packages
I work on, one of the issues I stumbled over is this code in this
`dlpath` method:
```julia
function dlpath(libname::Union{AbstractString, Symbol})
handle = dlopen(libname)
path = dlpath(handle)
dlclose(handle)
return path
end
```
The `dlopen` modified in this PR can in principle return `nothing`. But
there is no `dlpath` method for this. If one loads just a plain Julia,
all is fine, but under certain conditions (deep in a call chain analyzed
with Cthulhu.jl) it ended up not being able to prove that `path` will be
a string, and only inferred it as `Any`.
But if `throw_error` is set to `true` (the default, and used in the
relevant code path) then `dlopen` cannot return `nothing`, it always
returns a `String`. But the Julia compiler can't know that, as the
exception is thrown by a C helper. So instead modify the Julia code a
bit to help Julia deduce this fact by itself.
