Capgen needs a check to ensure the character length for `errmsg` is not being set by physics schemes

## Description
Over time we have decided to keep the length of `errmsg` in physics schemes unset, to be handled by the host instead:

**YES**
      `character(len=*), intent(out) :: errmsg`
      `character(len=*) :: errmsg`

**NO**
      `character(len=cx)        :: errmsg`

## Solution
Add a check in Capgen that will raise an error if schemes attempt to set the length of the `errmsg` variable