Fix GRPC 500 from BlobsResolver

What does this MR do and why?

This MR fixes a bug that occurred when an invalid ref was passed to BlobsResolver, which resulted in a GRPC::InvalidArgument being thrown and a 500 for the requester.

Now, BlobsResolver validates the ref and returns a GraphQL ArgumentError with the message Ref is not valid

Resolves: GRPC::InvalidArgumentGraphqlController#execute
Part of: #349973 (closed)

Screenshots or screen recordings

Before the fix:

GPRC exception causes 500

After the fix:

ArgumentError is returned

How to set up and validate locally

1. Go to /-/graphql-explorer
2. Enter the following query with your own project and file path:

   {
     project(fullPath: "PROJECT_FULL_PATH") {
       repository {
         blobs(paths: "REAL_FILE_PATH", ref: ":::") {
           nodes {
             id
           }
         }
       }
     }
   }

3. After submitting the query, see that you receive the following message:

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