Issue #377: Validate repository URLs

The repository URLs have a specific syntax and should point to a valid
destination. Currently, Kimchi does not check those constraints before
creating and updating a repository.

Add URL validation before creating and updating a repository. The URLs
must have one of the valid protocols (i.e. http://, https://, ftp://,
file://) and the URL address must point to a valid destination. That
means that if the URL protocol is file://, it should point to an
existing file on the system, otherwise it should point to an available
remote location.

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>
typo: Fix "repositorie"

The word "repositorie" is spelled wrong.

Fix the word by writing it correctly.

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>
repository: Remove error message prefix

If an error occurs when a repository is added or updated, the error
message has a prefix ("Failed.") which does not look good and does not
exist in other error messages.

Remove the message prefix and display only the reason which caused
the error.

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>