Internal server error when creating schema with no attributes

[nrempel]

When trying to create a schema, if you send a data structure with no attributes the server will respond with:

500 Internal Server Error

Server got itself in trouble

Server logs:

2020-05-21 17:47:32,561 aries_cloudagent.core.dispatcher ERROR Handler error: schemas_send_schema
Traceback (most recent call last):
  File "/home/indy/aries_cloudagent/issuer/indy.py", line 68, in create_and_store_schema
    origin_did, schema_name, schema_version, json.dumps(attribute_names),
  File "/home/indy/.pyenv/versions/3.6.9/lib/python3.6/site-packages/indy/anoncreds.py", line 71, in issuer_create_schema
    issuer_create_schema.cb)
indy.error.CommonInvalidStructure

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/indy/aries_cloudagent/ledger/indy.py", line 301, in create_and_send_schema
    public_info.did, schema_name, schema_version, attribute_names,
  File "/home/indy/aries_cloudagent/issuer/indy.py", line 68, in create_and_store_schema
    origin_did, schema_name, schema_version, json.dumps(attribute_names),
  File "/home/indy/aries_cloudagent/indy/error.py", line 27, in __exit__
    ) from err_value
aries_cloudagent.issuer.base.IssuerError: Error when creating schema: Error: Invalid structure
  Caused by: Empty list of Schema attributes has been passed


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/indy/aries_cloudagent/messaging/schemas/routes.py", line 110, in schemas_send_schema
    issuer, schema_name, schema_version, attributes
  File "/home/indy/aries_cloudagent/ledger/indy.py", line 304, in create_and_send_schema
    raise LedgerError(err.message) from err
aries_cloudagent.ledger.error.LedgerError: Error when creating schema: Error: Invalid structure
  Caused by: Empty list of Schema attributes has been passed

2020-05-21 17:47:32,563 aiohttp.server ERROR Error handling request
Traceback (most recent call last):
  File "/home/indy/aries_cloudagent/issuer/indy.py", line 68, in create_and_store_schema
    origin_did, schema_name, schema_version, json.dumps(attribute_names),
  File "/home/indy/.pyenv/versions/3.6.9/lib/python3.6/site-packages/indy/anoncreds.py", line 71, in issuer_create_schema
    issuer_create_schema.cb)
indy.error.CommonInvalidStructure

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/indy/aries_cloudagent/ledger/indy.py", line 301, in create_and_send_schema
    public_info.did, schema_name, schema_version, attribute_names,
  File "/home/indy/aries_cloudagent/issuer/indy.py", line 68, in create_and_store_schema
    origin_did, schema_name, schema_version, json.dumps(attribute_names),
  File "/home/indy/aries_cloudagent/indy/error.py", line 27, in __exit__
    ) from err_value
aries_cloudagent.issuer.base.IssuerError: Error when creating schema: Error: Invalid structure
  Caused by: Empty list of Schema attributes has been passed


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/indy/.pyenv/versions/3.6.9/lib/python3.6/site-packages/aiohttp/web_protocol.py", line 418, in start
    resp = await task
  File "/home/indy/.pyenv/versions/3.6.9/lib/python3.6/site-packages/aiohttp/web_app.py", line 458, in _handle
    resp = await handler(request)
  File "/home/indy/.pyenv/versions/3.6.9/lib/python3.6/site-packages/aiohttp/web_middlewares.py", line 119, in impl
    return await handler(request)
  File "/home/indy/.pyenv/versions/3.6.9/lib/python3.6/site-packages/aiohttp_apispec/middlewares.py", line 45, in validation_middleware
    return await handler(request)
  File "/home/indy/aries_cloudagent/admin/server.py", line 192, in apply_limiter
    return await task
  File "/home/indy/aries_cloudagent/messaging/schemas/routes.py", line 110, in schemas_send_schema
    issuer, schema_name, schema_version, attributes
  File "/home/indy/aries_cloudagent/ledger/indy.py", line 304, in create_and_send_schema
    raise LedgerError(err.message) from err
aries_cloudagent.ledger.error.LedgerError: Error when creating schema: Error: Invalid structure
  Caused by: Empty list of Schema attributes has been passed

It should instead catch the error and respond with a more helpful message and appropriate HTTP status code.

[sklump]

I'll take this among an overall error case polishing on messaging/schemas/.

[sklump]

Included in #527