Raise exception if zstandard module is not available

src/graph_tool/__init__.py

@@ -2869,7 +2869,7 @@ class Graph(object):
                 try:
                     cctx = zstandard.ZstdDecompressor()
                 except NameError:
-                    raise NotImplementedError("zstandard module not installed")
+                    raise NotImplementedError("zstandard module not installed, but it's required for decompression")
                 fctx = contextlib.ExitStack()
                 f = open(file_name, mode="rb")
                 fctx.enter_context(f)
@@ -2963,7 +2963,7 @@ class Graph(object):
                 try:
                     cctx = zstandard.ZstdCompressor(level=19)
                 except NameError:
-                    raise NotImplementedError("zstandard module not installed")
+                    raise NotImplementedError("zstandard module not installed, but it's required for compression")
                 fctx = contextlib.ExitStack()
                 f = open(file_name, mode="wb")
                 fctx.enter_context(f)

src/graph_tool/collection/netzschleuder.py

@@ -64,7 +64,10 @@ def get_ns_network(k, token=None):
         url = f"{url_prefix}/net/{k[0]}/files/{k[1]}.gt.zst"
 
     with open_ns_file(url, token) as f:
-        cctx = zstandard.ZstdDecompressor()
+        try:
+            cctx = zstandard.ZstdDecompressor()
+        except NameError:
+            raise NotImplementedError("zstandard module not installed, but it's required for zstd de-compression")
         with cctx.stream_reader(f) as fc:
             g = load_graph(fc, fmt="gt")