[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.

1 files changed, 20 insertions, 0 deletions

diff --git a/sql/core/src/test/resources/sql-tests/results/string-functions.sql.out b/sql/core/src/test/resources/sql-tests/results/string-functions.sql.out
new file mode 100644
index 0000000000..6961e9b659
--- /dev/null
+++ b/sql/core/src/test/resources/sql-tests/results/string-functions.sql.out
@@ -0,0 +1,20 @@
+-- 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