python: Add a warning if pydot is not available.

Silently failing makes it hard to debug what happened. Add a warning.

Change-Id: Ia61b8de937bb254898726ad551fb5c894104d771
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26045
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/m5/util/dot_writer.py

@@ -338,6 +338,8 @@ def dot_create_dvfs_nodes(simNode, callgraph, domain=None):
 
 def do_dot(root, outdir, dotFilename):
     if not pydot:
+        warn("No dot file generated. " +
+             "Please install pydot to generate the dot file and pdf.")
         return
     # * use ranksep > 1.0 for for vertical separation between nodes
     # especially useful if you need to annotate edges using e.g. visio
@@ -358,6 +360,8 @@ def do_dot(root, outdir, dotFilename):
 
 def do_dvfs_dot(root, outdir, dotFilename):
     if not pydot:
+        warn("No dot file generated. " +
+             "Please install pydot to generate the dot file and pdf.")
         return
 
     # There is a chance that we are unable to resolve the clock or