Add a set of error classes

repo_management/errors.py:
Add the Exception class `RepoManagementError` serving as a base, from
which `RepoManagementFileError` is derived, which serves as a general
Error class to signal problems with file interaction.
Add the Exception class `RepoManagementFileNotFoundError`, which is
derived from `RepoManagementFileError`.
Add `RepoManagementValidationError` which derives from
`RepoManagementError` and is raised on validation issues of file
data.

repo_management/errors.py

+class RepoManagementError(Exception):
+    """A class of Errors that is raised on issues with handling a repository database"""
+
+
+class RepoManagementFileError(RepoManagementError):
+    """An Error that is raised on issues with reading or writing files using repo_managment"""
+
+
+class RepoManagementValidationError(RepoManagementError):
+    """An Error that is raised on issues with validating files using repo_managment"""
+
+
+class RepoManagementFileNotFoundError(RepoManagementFileError, FileNotFoundError):
+    """An Error that is raised when a file can not be found"""