Calling hermes add keys can overwrite a key without any confirmation or warning

[AlianBenabdallah]

Summary of Bug

It is possible to overwrite an existing key with hermes add without any confirmation or warning. It would make sense that an add subcommand would only add a key and not modify one, potentially by mistake.

Version

v0.15.0 (guide version)
Possibly present in every version

Steps to Reproduce

gaiad keys add testkey --output json > key_seed.json
gaiad keys add testkey --output json > key_seed2.json
// Note that the two keys are different except for their name.
hermes keys add ibc-0 -f key_seed.json
hermes keys add ibc-0 -f key_seed2.json

Acceptance Criteria

hermes keys add issues a warning if a key already exists and ask for confirmation or a new command hermes modify.

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