Improve error messages on arrays with negative length

[llogiq]

Describe the bug The error message on specifying a negative-length array, while not wrong, could be more helpful (and suggest specifying a positive length instead)

To Reproduce Steps to reproduce the behavior:

1. Schema (if applicable)

   {
   "type": "array",
   "length": -1,
   "content": {
       "type": "object"
   }
   }

2. See error

   BadRequest: could not convert from value 'i64(-1)': Type { expected: "U32", got: "i64(-1)" }

Expected behavior

Something like "you specified an array of negative length (-1). Arrays can only have length 0 or higher. Did you mean length 1?"

Environment (please complete the following information):

• OS: any
• Version: 0.5.6