Document error behavior of audb.Dependencies methods

[hagenw]

As stated in #354, the following methods all raise a KeyError at the moment if a file is requested that is not part of the deopendency table:

• audb.Dependencies.__init__()
• audb.Dependencies.archive()
• audb.Dependencies.bit_depth()
• audb.Dependencies.channels()
• audb.Dependencies.checksum()
• audb.Dependencies.duration()
• audb.Dependencies.format()
• audb.Dependencies.removed()
• audb.Dependencies.sampling_rate()
• audb.Dependencies.type()
• audb.Dependencies.version()

We need to decide if we want to stick with this error, and add "Raises" sections to the docstrings of the methods.