Improve error message for private key not found

[jribbink]

Issue To Be Solved

When developers new to Flow fork a starter repo, it's very likely that they will encounter errors relating to their private key not being found. To someone without context, the error could be pretty confusing:

❌ Failed to deploy contract ExampleConnectors: failed to deploy contract ExampleConnectors: could not load the key for the account from provided location testnet.pkey: open testnet.pkey: no such file or directory
❌ Command Error: failed deploying all contracts

(Optional): Suggest A Solution

Adjust the error message to give user clear resolution actions & be more descriptive

(Optional): Context

n/a