Issue title: Negative diameter does not produce an error message
 Issue body: Click on a tree that has a valid diameter, and Edit Details.

Change the diameter to a negative number and Save. It refreshes into display mode with the original diameter.

It ought to produce an error message and remain in edit mode.
 Comments: 
 Comment 0: I just attempted to replicate this, and encountered a different bug.

I entered `-2` as the diameter, and it was saved as `2` instead.
 Comment 1: I replicated this bug where the negative diameter saves as the positive version of the number.
{'source': 'GithubIssues', 'repo_name_with_owner': 'OpenTreeMap/otm-core', 'issue_id': 175330807, 'repo_stars': 129.0}