[SPARK-18430][SQL] Fixed Exception Messages when Hitting an Invocation Exception of Function Lookup

### What changes were proposed in this pull request?
When the exception is an invocation exception during function lookup, we return a useless/confusing error message:

For example,
```Scala
df.selectExpr("concat_ws()")
```
Below is the error message we got:
```
null; line 1 pos 0
org.apache.spark.sql.AnalysisException: null; line 1 pos 0
```

To get the meaningful error message, we need to get the cause. The fix is exactly the same as what we did in https://github.com/apache/spark/pull/12136. After the fix, the message we got is the exception issued in the constuctor of function implementation:
```
requirement failed: concat_ws requires at least one argument.; line 1 pos 0
org.apache.spark.sql.AnalysisException: requirement failed: concat_ws requires at least one argument.; line 1 pos 0
```

### How was this patch tested?
Added test cases.

Author: gatorsmile <gatorsmile@gmail.com>

Closes #15878 from gatorsmile/functionNotFound.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -446,7 +446,10 @@ object FunctionRegistry {
         // If there is an apply method that accepts Seq[Expression], use that one.
         Try(varargCtor.get.newInstance(expressions).asInstanceOf[Expression]) match {
           case Success(e) => e
-          case Failure(e) => throw new AnalysisException(e.getMessage)
+          case Failure(e) =>
+            // the exception is an invocation exception. To get a meaningful message, we need the
+            // cause.
+            throw new AnalysisException(e.getCause.getMessage)
         }
       } else {
         // Otherwise, find a constructor method that matches the number of arguments, and use that.

sql/core/src/test/resources/sql-tests/inputs/string-functions.sql

+-- Argument number exception
+select concat_ws();
+select format_string();

sql/core/src/test/resources/sql-tests/results/string-functions.sql.out

+-- Automatically generated by SQLQueryTestSuite
+-- Number of queries: 2
+
+
+-- !query 0
+select concat_ws()
+-- !query 0 schema
+struct<>
+-- !query 0 output
+org.apache.spark.sql.AnalysisException
+requirement failed: concat_ws requires at least one argument.; line 1 pos 7
+
+
+-- !query 1
+select format_string()
+-- !query 1 schema
+struct<>
+-- !query 1 output
+org.apache.spark.sql.AnalysisException
+requirement failed: format_string() should take at least 1 argument; line 1 pos 7