Export conversion error ErrorUnexpectedTypeConversion (#1227)

* Export conversion error

* Remove redundant line

* Fix name of error

data/sqlutil/converter.go

@@ -2,6 +2,7 @@ package sqlutil
 
 import (
 	"database/sql"
+	"errors"
 	"fmt"
 	"reflect"
 	"regexp"
@@ -11,6 +12,8 @@ import (
 	"github.com/grafana/grafana-plugin-sdk-go/data"
 )
 
+var ErrorUnexpectedTypeConversion = errors.New("conversion error")
+
 // FrameConverter defines how to convert the scanned value into a value that can be put into a dataframe (OutputFieldType)
 type FrameConverter struct {
 	// FieldType is the type that is created for the dataframe field.
@@ -451,5 +454,6 @@ var TimeToNullableTime = data.FieldConverter{
 }
 
 func toConversionError(expected string, v interface{}) error {
-	return fmt.Errorf(`expected %s input but got type %T for value "%v"`, expected, v, v)
+	return fmt.Errorf(`%w, expected %s input but got type %T for value "%v"`,
+		ErrorUnexpectedTypeConversion, expected, v, v)
 }