## What changes were proposed in this pull request?
This PR proposes to fix the error message when some data types are compatible and others are not in set/union operation.
Currently, the code below:
```scala
Seq((1,("a", 1))).toDF.union(Seq((1L,("a", "b"))).toDF)
```
throws an exception saying `LongType` and `IntegerType` are incompatible types. It should say something about `StructType`s with more readable format as below:
**Before**
```
Union can only be performed on tables with the compatible column types.
LongType <> IntegerType at the first column of the second table;;
```
**After**
```
Union can only be performed on tables with the compatible column types.
struct<_1:string,_2:string> <> struct<_1:string,_2:int> at the second column of the second table;;
```
*I manually inserted a newline in the messages above for readability only in this PR description.
## How was this patch tested?
Unit tests in `AnalysisErrorSuite`, manual tests and build wth Scala 2.10.
Author: hyukjinkwon <gurwls223@gmail.com>
Closes#16882 from HyukjinKwon/SPARK-19544.
4321ff9edd