Clarify the error in the Inter-Branch merge flow when syntax error

[f-alizada]

• [ ] This issue is blocking
• [x] This issue is causing unreasonable pain

Context

There are cases when the the configuration of the merge-flow file is invalid jsonc The only error we get: WARNING: Failed to fetch and process configuration file and the action completes with success.

Example of such case: https://github.com/dotnet/msbuild/pull/11190

Suggestion to separate the networking and invalid json format cases for proper reporting.

[missymessa]

@mmitche which team owns this functionality?

[mmitche]

We do :)

[missymessa]

@premun is this a mirror issue?

[premun]

@missymessa not us