check content-type on handleAdd early

[anjor]

I was trying to upload a large dataset and it failed after nearly 7 minutes because I had the wrong content-type in my request:

root@c3-small-x86-01:/data# time curl https://upload.estuary.tech/content/add -H "Authorization: Bearer $(cat /data/.estuary_auth)" -H "Content-Type: multipar/form-data" -F "data=@longitudinal-nutrient-deficiency.tar"
{"error":{"code":500,"reason":"Internal Server Error","details":"request Content-Type isn't multipart/form-data"}}

real	6m52.221s
user	0m15.953s
sys	0m18.861s

Checking it early to handle similar instances.

[en0ma]

LGTM