String type label cause sqlflowserver process failed

[BlackPoint-CX]

Description
Wrongly defined string type of label will cause sqlflowserver crush down and throws error.

FATA[0549] Unsupported label data type: STRING_TYPE      package=sql 

[tonyyang-svail]

@BlackPoint-CX Thanks for submitting this PR. You are right. The server should return an error instead of crushing down.