How to implement a (derived) Exception type in Julia?

I'm trying to figure out how to implement a custom Exception type for a library which I am writing in Julia.

This is what I have come up with so far, mostly by following information I have found elsewhere online.

struct LibraryException <: Exception
    message::String
end

Base.showerror(io::IO, err::LibraryException) = print(io, err.message)

This seems to be common across the few resources which I have found, however each reference diverges in one way or another. The inconsistency between each reference does not give me a high degree of confidence in the accuracy of the information.

The existing code which I have looks a bit like this.

try
    return someFunction(someargs)
catch err
    if (err isa ErrorException) && (occursin("part of expected message", sprint(showerror, err)))
        printlnOrLog(err)
        return false
    else
        rethrow(err)
    end
end

Which would be better if it was like this.

catch err
    if err isa LibraryException

This is correct. Once someFunction stops using throw("random string") and uses throw(LibraryException("random string")), if err isa LibraryException will work.