[SPARK-8072] [SQL] Better AnalysisException for writing DataFrame with identically named columns

Adding a function checkConstraints which will check for the constraints to be applied on the dataframe / dataframe schema. Function called before storing the dataframe to an external storage. Function added in the corresponding datasource API.
cc rxin marmbrus

Author: animesh <animesh@apache.spark>

This patch had conflicts when merged, resolved by
Committer: Michael Armbrust <michael@databricks.com>

Closes #7013 from animeshbaranawal/8072 and squashes the following commits:

f70dd0e [animesh] Change IO exception to Analysis Exception
fd45e1b [animesh] 8072: Fix Style Issues
a8a964f [animesh] 8072: Improving on previous commits
3cc4d2c [animesh] Fix Style Issues
1a89115 [animesh] Fix Style Issues
98b4399 [animesh] 8072 : Moved the exception handling to ResolvedDataSource specific to parquet format
7c3d928 [animesh] 8072: Adding check to DataFrameWriter.scala