ENH: Use appropriate message when reporting TCK file attribute error

Use appropriate message when reporting `file` attribute error in TCK:
the previous message was mistakenly reporting the non-existence of the
`file` attribute, which indeed has to exist to be able to do the
`hdr['file']` without getting a `KeyError`. This patch set uses the
message that was not being used when the check was not being successful.

Author: jhlegarreta

nibabel/streamlines/tck.py

@@ -385,7 +385,7 @@ def _read_header(cls, fileobj):
                 'TCK only supports single-file - in other words the filename part must be '
                 f"specified as '.' but '{hdr['file'].split()[0]}' was specified."
             )
-            raise HeaderError("Missing 'file' attribute in TCK header.")
+            raise HeaderError(msg)
 
         # Set endianness and _dtype attributes in the header.
         hdr[Field.ENDIANNESS] = '>' if hdr['datatype'].endswith('BE') else '<'