ProcessGraphMissing error is bit confusing

[soxofaan]

minor issue:

openeo-api/errors.json

Lines 194 to 196 in f303d65

	"ProcessGraphMissing": {
	"description": "The parameter `process` doesn't contain a valid process.",
	"message": "Invalid process specified.",

"process graph is missing" (error code) and "process graph is invalid" (description/message) are not the same problem.
While you could argue, strictly speaking, that they are technically equivalent, for the end user there is a subtle difference:

• missing: there is nothing
• invalid: there is something, but it's invalid