Add context to missing key file error

[madisoncarter1234]

Summary

• Adds path context to the error when a key file cannot be read
• Fixes Add more context to missing key error #2059

Before:

Error: No such file or directory (os error 2)

After:

Error: failed to read key file: /path/to/keys.env

Caused by:
    No such file or directory (os error 2)

Test plan

• cargo check -p sequencer passes
• Manual verification: attempt to start sequencer with non-existent key file path