Proposed changes:
* Clarify the type error that `Column.substr()` gives.

Test plan:
* Tested this manually.
* Test code:
    ```python
    from pyspark.sql.functions import col, lit
    spark.createDataFrame([['nick']], schema=['name']).select(col('name').substr(0, lit(1)))
    ```
* Before:
    ```
    TypeError: Can not mix the type
    ```
* After:
    ```
    TypeError: startPos and length must be the same type. Got <class 'int'> and
    <class 'pyspark.sql.column.Column'>, respectively.
    ```

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #18926 from nchammas/SPARK-21712-substr-type-error.