channels: Strictly check the version of '.guix-channel'.

channels: Strictly check the version of '.guix-channel'.

Until now the 'version' field in '.guix-channel' could be omitted, or it
could be any value.

* guix/channels.scm (read-channel-metadata): Rename to...
(channel-instance-metadata): ... this.
(channel-instance-dependencies): Adjust accordingly.
(read-channel-metadata): New procedure.  Use 'match'
to require a 'version' field.  Provide proper error handling when the
channel sexp is malformed or when given an unsupported version number.
(read-channel-metadata-from-source): Use 'catch' and
'system-error-errno' instead of 'file-exists?'.
* tests/channels.scm (instance--unsupported-version): New variable.
(read-channel-metadata): Rename to...
(channel-instance-metadata): ... this.  Rename tests accordingly.
("channel-instance-metadata rejects unsupported version"): New test.