Better exception descriptions

[antonioan]

Is your feature request related to a problem? Please describe.

1. Trying to name a function get fails with

pdl.pdl_ast.PDLRuntimeError: Type error: ${utils.get} is of type <class 'builtin_function_or_method'> but should be a function.

2. Accidentally indenting the with: when match: is empty, i.e.,

  match:
    with:
# ^^ <- wrong 
      - if: condition
        then: result
      - then: default

fails with the vague exception:

['utils.pdl:0 - Missing required field: function', 'utils.pdl:0 - Missing required field: return', "utils.pdl:11 - {'temperature': '${temperature}', 'top_p': '${top_p}'} should be of type: {'anyOf': [{'$ref': '#/$defs/LocalizedExpression_TypeVar_'}... (truncated)

3. Accidentally using the keyword dict instead of object to describe an (optional) function parameter type fails with the same vague exception above.

Describe the solution you'd like

A clearer and more concise description of what went wrong.