Improve error handling of parameters

[kaklakariada]

When self._parameters in MessageExpander has an invalid type (e.g. string), the expand() method will fail with the following error:

/Users/user/.luarocks/share/lua/5.4/MessageExpander.lua:37: bad argument #1 to 'next' (table expected, got string)

stack traceback:
        /Users/chp/.luarocks/share/lua/5.4/MessageExpander.lua:38: in function 'MessageExpander.expand'
        (...tail calls...)
        /Users/chp/.luarocks/share/lua/5.4/ExaError.lua:27: in function 'ExaError.__tostring'

See code: https://github.com/exasol/error-reporting-lua/blob/main/src/MessageExpander.lua#L37

Instead of failing, the code should check for the correct type before calling next().