create branch name validation message

[eden-ohana]

fixes #2894

argument branch: branch id: invalid value: validation error can contain only letters, numbers, underscores, and dashes (not at the beginning)

[arielshaqed]

This will have the form

branch id: invalid value: can contain only letters, numbers, underscores, and dashes (not at the beginning)

which looks somewhat different from ErrInvalidRef and friends. Consider

Suggested change

	ErrInvalidBranchID = fmt.Errorf("branch id: %w can contain only letters, numbers, underscores, and dashes (not at the beginning)", ErrInvalidValue)
	ErrInvalidBranchID = fmt.Errorf("branch id must consist of letters, digits, underscores and dashes, and cannot start with a dash: %w", ErrInvalidValue)

However I would also like to question whether we need this validation almost anywhere except during creation. Specifically, if I try to create a branch named '-not=valid--` then it should fail. But anywhere else I only fetch a branch, and I think I should be just as happy to be told "branch not found".

[eden-ohana]

message: "argument branch: branch id: invalid value: validation error: branch id must consist of letters, digits, underscores and dashes, and cannot start with a dash"

returns only on create branch validation