Bad error message inserting when no Schema Registry connectivity

[MichaelDrogalis]

Describe the bug

When inserting a record into a stream that's serialized with Avro, a poor error message is raised if ksqlDB can't connect to Schema Registry.

To Reproduce

With ksqlDB 0.6.0, where Schema Registry is not configured on the server:

CREATE STREAM s1 (c1 varchar) WITH (kafka_topic='t1', partitions=1, key='c1', value_format = 'avro');

insert into s1 (c1) values ('a');

Expected behavior

An error message indicating that ksqlDB couldn't connect to Schema Registry.

Actual behaviour

Failed to insert values into 'S1'. Could not serialize row: [ 'a' ]

This error message is tough because in a more complex setting, it leads the user to believe that it's a problem with their data.

[stevenpyzhang]

@MichaelDrogalis this should be fixed with #4325
The error message should now say something like:

Caused by: KSQL is not configured to use a schema registry. To enable it, please
        set ksql.schema.registry.url

[MichaelDrogalis]

@stevenpyzhang Nice one. Closing, staged for 0.7.