Fix initial token loading

[Yohan460]

Currently token bootstrapping with the file backend is non-functional. This PR resolves that issue

When performing the token upload operation you'd get one of the two errors

{
  "error": "retrieving auth tokens: open db/nanomdm-dev.tokens.json: no such file or directory"
}

or

{
  "error": "mismatched consumer key"
}

This PR adds some error validation, and default value checking to resolve this issue

[jessepeterson]

This is a good catch, not sure how I missed this. Obviously we can't err on retrieving the tokens first because it's a proper error if they're not yet set/exist yet. I think the correct fix is something like this (i.e. eat the error):

diff --git a/http/api/ckcheck.go b/http/api/ckcheck.go
index 5288c00..1ee4606 100644
--- a/http/api/ckcheck.go
+++ b/http/api/ckcheck.go
@@ -3,7 +3,6 @@ package api
 import (
        "context"
        "errors"
-       "fmt"
 
        "github.com/micromdm/nanodep/client"
 )
@@ -32,10 +31,7 @@ func NewCKCheck(store AuthTokensStore) *CKCheck {
 // the provided auth tokens.
 func (t *CKCheck) StoreAuthTokens(ctx context.Context, name string, tokens *client.OAuth1Tokens) error {
        prevTokens, err := t.AuthTokensRetriever.RetrieveAuthTokens(ctx, name)
-       if err != nil {
-               return fmt.Errorf("retrieving auth tokens: %w", err)
-       }
-       if prevTokens.ConsumerKey != tokens.ConsumerKey {
+       if err == nil && prevTokens != nil && prevTokens.ConsumerKey != tokens.ConsumerKey {
                return CKMismatch
        }
        return t.AuthTokensStorer.StoreAuthTokens(ctx, name, tokens)

The longer term fix, however, since I've discovered some cases where the Consumer Key can legitimately change is probably to add capability for a "force" mode which will override the tokens even if the CK changed.