Rescue the correct error when fetching credentials

In commit 08df3056, the `LoadError` class was changed such that it
inherits from `::Fog::Errors::Error` rather than Ruby's built-in
`LoadError` object, as it was previously.

That commit omitted to update this `rescue` line, meaning that
`::Fog::Errors::LoadError` was not being rescued.
Merge pull request #97 from mattbostock/83470576-fix_rescue_load_error

[#83470576] Rescue the correct error when fetching credentials