[SPARK-13282][SQL] LogicalPlan toSql should just return a String

Previously we were using Option[String] and None to indicate the case when Spark fails to generate SQL. It is easier to just use exceptions to propagate error cases, rather than having for comprehension everywhere. I also introduced a "build" function that simplifies string concatenation (i.e. no need to reason about whether we have an extra space or not).

Author: Reynold Xin <rxin@databricks.com>

Closes #11171 from rxin/SPARK-13282.