fix: Better errors on over length persona/human files

[cpacker]

Please describe the purpose of this pull request.

Before:

% memgpt run --persona too_long

🧬 Creating new agent...
->  🤖 Using persona profile 'too_long'
->  🧑 Using human profile 'cs_phd'
🎉 Created new agent 'agent_4'

Traceback (most recent call last):
...
ValueError: Edit failed: Exceeds 2000 character limit (requested 22073). Consider summarizing existing core memories in 'persona' and/or moving lower priority content to archival memory to free up space in core memory, then trying again.

After:

 % memgpt run --persona too_long

🧬 Creating new agent...
->  🤖 Using persona profile 'too_long'
->  🧑 Using human profile 'cs_phd'
Failed to create agent from provided information:
Contents of too_long.txt is over the character limit (22073 > 2000)