graphviz errors ignored without logging

[Hal]

ticket:7415 shows a case where it was difficult to debug, as the error is silently ignored.

In this case, the error comes from execution of an external program (dot) via the subprocess.Popen.communicate method

It would be helpful to log, at DEBUG level:

• return code (at least if non-zero)
• stderr output

My personal preference would be for all other functionality of master ticket to continue to work, but raising an exception is also reasonable.

[Hal]

Patch to add logging of dot command errors

[Ryan J Ollos]

Patch can be simplified a bit, but otherwise looks good.

[Ryan J Ollos]

Example of error message:

08:44:06 PM Trac[graphviz] ERROR: dot dot failed with code 1: Format:
"png2" not recognized. Use one of: canon cmap cmapx cmapx_np dot eps fig
gd gd2 gif gv imap imap_np ismap jpe jpeg jpg pdf plain plain-ext png ps
ps2 svg svgz tk vml vmlz vrml wbmp x11 xdot xlib

[Ryan J Ollos]

(In [12946]) Refs #7621: Log errors from calling GraphViz.