improve error message for invalid branch name

[ceedubs]

This error message doesn't make it clear that it didn't like the . characters in my branch name.

nimbus/main> branch cloud-client-6.0.0 
 
  branch (or branch.create, create.branch)
  `branch foo`       forks the current project branch to a new branch `foo`
  `branch /bar foo`  forks the branch `bar` of the current project to a new branch `foo`
  `branch .bar foo`  forks the path `.bar` of the current project to a new branch `foo`

nimbus/main> branch cloud-client-6-0-0
 
  Done. I've created the cloud-client-6-0-0 branch based off of main.
  
  Tip: Use `merge /cloud-client-6-0-0 /main` to merge your work back into the main branch.

[ceedubs]

I just ran into this again. This time apparently because my branch name started with a digit:

@unison/cloud/main> branch 401-msg
 
  branch (or branch.create, create.branch)
  `branch foo`       forks the current project branch to a new branch `foo`
  `branch /bar foo`  forks the branch `bar` of the current project to a new branch `foo`
  `branch .bar foo`  forks the path `.bar` of the current project to a new branch `foo`

@unison/cloud/main> branch msg-401
 
  Done. I've created the msg-401 branch based off of main.
  
  Tip: Use `merge /msg-401 /main` to merge your work back into the main branch.