Exception thrown when no tables exist, improve error message about respawn

[lesair]

@dennisroche , I stumbled upon this as exception as well while trying to Checkpoint.Reset an SQL database in Azure I forgot to create its tables. My first thought was "please don't tell me Respawn has issues with SQL databases in Azure", but then I discarded that thought because it really made no sense, and started digging into the aforementioned exception, until I reached this issue and I realized I forgot to initialize my database.

I believe a more meaningful error message or exception would have led me to the root cause faster.

[Inspyro]

As a workaround you can decorate the DbAdapter like so:

new Checkpoint
{
  SchemasToInclude = ..., TablesToIgnore = ...
  DbAdapter = new EmptyCommandHandlingDbAdapterDecorator(DbAdapter.SqlServer)
}

and override the BuildDeleteCommandText method:

public string BuildDeleteCommandText (GraphBuilder builder)
{
  var command = _dbAdapter.BuildDeleteCommandText(builder);

  if (string.IsNullOrEmpty(command))
    throw new NothingToClearException();

  return command;
}

but I would also appreciate that this bug was being fixed by throwing a better exception out of the box

[majouleblanc]

this issue still happening on v5.0.1, i managed to work around it by just replacing
await Fixture.Checkpoint.Reset(Database.ConnectionStringAdmin);
by
VotingContext.Votes.RemoveRange(VotingContext.Votes);
await VotingContext..SaveChangesAsync();

VotingContext.ChangeTracker.Entries()
.Where(e => e.Entity != null).ToList()
.ForEach(e => e.State = EntityState.Detached);

[jbogard]

PR welcome!