Improve error message when AvalancheGo binary not found

[martineckardt]

Currently the user receives the following error message:

Error: rpc error: code = Unknown desc = avalanche exec not exists

A user getting started with Avalanche Network Runner does not understand the issue or the solution:

Possible Solution:
Enrich the error message:

• Explain issue more clearly: "AvalancheGo could no be located in the directory you have provided as the flag --avalanche-exec or environment variable AVALANCHEGO_EXEC_PATH"
• Explain prerequisite: "AvalancheGo binary needs to be located in the specified directory."
• Explain Solution: "Download the AvalancheGo binary from github and provide the path to the binary when starting the network"