WMS Server crashes if (some) files do not contain a variable associated with a dimension

[Joern Ungermann]

E.g. a file contains a "level" dimension, but no "level" variable.

Desired behaviour: Do not crash, provide error message, skip affected file.

[Reimar Bauer]

Added a check on presence of coordinate variables in NetCDF file

Previously, a KeyError was raised, which crashed the Server. The IOError provides an error message to the Client, helping debugging tremendously. Also the error message is more explicit.

Resolve issue #220

→ <<cset 942fe7e36952>>