Detect and report errors reading mergeable sources

[hlship]

• Please check if the PR fulfills these requirements

• [ x] There is an open issue describing the problem that this pr intents to solve.
  • [x ] You have a descriptive commit message with a short title (first line).
• [ x] Tests for the changes have been added (for bug fixes / features).
• Docs have been added / updated (for bug fixes / features).

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Bug fix for #42

• What is the current behavior?

The original exception is reported but gives no context about what file contains the parse error.

• What is the new behavior (if this is a feature change)?

Catches exceptions when parsing mergeable files, and throws a new exception with details about what file was being read when the original exception occured.

Also, removed the spinner output, which makes reading log ouput harder.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Not a breaking change.

• Other information:

[alan-ghelardi]

Thank you again @hlship!