Return exceptions for all filesystem errors

[mcrumm]

The following changes are aimed at simplifying the Briefly API:

Briefly.create() and Briefly.create(opts) now return either {:ok, path} or {:error, Exception.t()}. The following exceptions may be returned:

• %Briefly.NoRootDirectoryError{} - Returned if Briefly is unable to create a root temporary directory.

• %Briefly.WriteError{} - Returned when a temporary entry cannot be created. The exception contains the POSIX error code the caused the failure.

The :too_many_attempts error is not directly actionable so it is no longer surfaced. Since Briefly still performs several write attempts when faced with :eexist and :eaccess error codes, too many attempts can be inferred when either of those codes are returned.

The error messages raised by Briefly.create!() have been moved to the Exception modules and aside from no longer printing the number of attempts they provide the same information.

For example, if you have this:

case Briefly.create() do
  {:ok, path} -> path
  {:no_space, _} -> raise "no space"
  {:too_many_attempts, _} -> raise "too many attempts"
  {:no_tmp, _} -> raise "no tmp dirs"
end

...then change it to this:

case Briefly.create() do
  {:ok, path} -> path
  {:error, %Briefly.WriteError{code: :enospc}} -> raise "no space"
  {:error, %Briefly.WriteError{code: c}} when c in [:eexist, :eacces] -> raise "too many attempts"
  {:error, %Briefly.NoRootDirectoryError{}} -> raise "no tmp dirs"
  {:error, exception} -> raise exception
end

[benwilson512]

Definitely a nice improvement, thanks!