command: exit non-zero if uploading to HCP failed

In the current state, a Packer build that succeeds but fails to push its
metadata to HCP for reasons other than a lack of artifact will always
succeed from the perspective of a user invoking `packer build`.

This can be a bit misleading, as users may expect their artifacts to
appear on HCP Packer if their build succeeded on Packer Core, so this
commit changes this behaviour, instead reporting HCP errors as a real
error if the build failed, so packer returns a non-zero error code if
this happens.

command/build.go

@@ -315,6 +315,15 @@ Check that you are using an HCP Ready integration before trying again:
 					artifacts.Unlock()
 				}
 			}
+
+			// If the build succeeded but uploading to HCP failed,
+			// Packer should exit non-zero, so we re-assign the
+			// error to account for this case.
+			if hcperr != nil && err == nil {
+				errs.Lock()
+				errs.m[name] = hcperr
+				errs.Unlock()
+			}
 		}()
 
 		if cla.Debug {