Wrong exception message for unsupported INT96 type

[asfimport]

In AvroSchemaConverter, the message says "INT64 not yet implemented".

Reporter: Cheng Lian / @liancheng
Assignee: Cheng Lian / @liancheng

PRs and other links:

• PR #174

_{Note: This issue was originally created as PARQUET-254. Please see the migration documentation for further details.}

[asfimport]

Ryan Blue / @rdblue:
Merged #174