Adds error if user attempts to create an index in DB > 0.

[simonprickett]

Adds an error if the user attempts to create a search index on database > 0 as RediSearch does not support this.

Closes #190

[simonprickett]

Example of the error you will see now:

from redis_om import HashModel, Field, Migrator

class Person(HashModel):
    name: str = Field(index=True)
    age: int = Field(index=True)


simon = Person(
    name = "Simon",
    age = 99
)

simon.save()

Migrator().run()

results = Person.find(Person.name == "Simon").all()

print(results)

$ export REDIS_OM_URL=redis://127.0.0.1:6379/2
(venv) python_om_db_test $ python testapp.py
Traceback (most recent call last):
  File "/Users/simonprickett/tmp/python_om_db_test/testapp.py", line 15, in <module>
    Migrator().run()
  File "/Users/simonprickett/tmp/python_om_db_test/venv/lib/python3.9/site-packages/redis_om/model/migrations/migrator.py", line 165, in run
    migration.run()
  File "/Users/simonprickett/tmp/python_om_db_test/venv/lib/python3.9/site-packages/redis_om/model/migrations/migrator.py", line 76, in run
    self.create()
  File "/Users/simonprickett/tmp/python_om_db_test/venv/lib/python3.9/site-packages/redis_om/model/migrations/migrator.py", line 82, in create
    create_index(self.redis, self.index_name, self.schema, self.hash)
  File "/Users/simonprickett/tmp/python_om_db_test/venv/lib/python3.9/site-packages/redis_om/model/migrations/migrator.py", line 45, in create_index
    raise MigrationError(
redis_om.model.migrations.migrator.MigrationError: Creating search indexes is only supported in database 0. You attempted to create an index in database 2

[codecov-commenter]

Codecov Report

Merging #196 (6193c23) into main (c4e96b1) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #196      +/-   ##
==========================================
- Coverage   77.32%   77.30%   -0.03%     
==========================================
  Files          12       12              
  Lines        1160     1163       +3     
==========================================
+ Hits          897      899       +2     
- Misses        263      264       +1     

Flag	Coverage Δ
unit	77.30% <66.66%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aredis_om/model/migrations/migrator.py	75.86% <66.66%> (-0.33%)	⬇️

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