fix: don't assume that credential providers always return credentials or error during handshake.

The type-system allows this even though when getting credentials this should never happen and won't
in the default implementation. However, users can of course return nothing and provide no error which
is when we have to create one, instead of panicking.

Author: Byron

gix-protocol/src/handshake/function.rs

@@ -46,7 +46,7 @@ where
                 progress.set_name("authentication");
                 let credentials::protocol::Outcome { identity, next } =
                     authenticate(credentials::helper::Action::get_for_url(url.clone()))?
-                        .expect("FILL provides an identity or errors");
+                        .ok_or(Error::EmptyCredentials)?;
                 transport.set_identity(identity)?;
                 progress.step();
                 progress.set_name("handshake (authenticated)");

gix-protocol/src/handshake/mod.rs

@@ -68,6 +68,8 @@ mod error {
     pub enum Error {
         #[error("Failed to obtain credentials")]
         Credentials(#[from] credentials::protocol::Error),
+        #[error("No credentials were returned at all as if the credential helper isn't functioning unknowingly")]
+        EmptyCredentials,
         #[error("Credentials provided for \"{url}\" were not accepted by the remote")]
         InvalidCredentials { url: BString, source: std::io::Error },
         #[error(transparent)]