Remote: Set an error when a remote cannot be found.

Inside `git_remote_load`, the calls to `get_optional_config` use
`giterr_clear` to unset any errors that are set due to missing config
keys. If neither a fetch nor a push url config was found for a remote,
we should set an error again.
Formatting fixes for the docs

These are some issues I found while playing around with the new parser
for docurium.
index: split GIT_IDXENTRY into two flag enums

The documentation has shown this as a single enum for a long time. These
should in fact be two enums. One with the bits for the flags and another
with the bits for the extended flags.
Merge pull request #2396 from libgit2/cmn/docstuff

Documentation formatting fixes
Merge pull request #2389 from arthurschreiber/arthur/set-error-when-no-remote-found

Remote: Set an error when a remote cannot be found.