Fix error message on client.toml absence

[syvb]

Pull request checklist

• All tests pass. Demo project builds and runs.
• I have resolved any merge conflicts.

What's in this pull request?

This fixes the error message that occurs when client.toml isn't found. Before, it incorrectly referred to the location where the config file was searched for as where an example was. This fixes it so it correctly labels the expected config location, and includes a reference to the actual example config.

[fiaxh]

I'd suggest to leave a recommended action in the message to give the user a clear guidance on what to do. The full version of a recommended action could be along the lines of "Please create one at {} or specify a location using -c path/to/config.toml". You can use the path_example variable to refer to the location of the example config.

[syvb]

@fiaxh I've updated the error message to be like Config file not found. Please create one at /home/smit/.nimiq/client.toml or specify a location using -c path/to/config.toml, see the example config file at /home/smit/.nimiq/client.toml.example