Improve unit testing coverage in Fetcher.go

**What would you like to be added**:
Unit test coverage is currently significantly lacking in the public methods under fetcher.go, as discussed in https://github.com/aws/eks-anywhere/pull/1577. We need to add tests for the currently untested methods

**Why is this needed**:
Adding unit tests is good practice to avoid unintentional regressions

Start by reading fetcher.go and the discussion in pull request #1577 to identify the public methods that lack coverage. Add unit tests for those methods, then run the relevant Go package tests and confirm the untested public methods are covered.

Written by the indexing model from the issue text.