def call_graphviz_dot(src, fmt):
try:<EOL><INDENT>svg = dot(src, T=fmt)<EOL><DEDENT>except OSError as e:  <EOL><INDENT>if e.errno == <NUM_LIT:2>:<EOL><INDENT>cli.error("""<STR_LIT>""")<EOL><DEDENT>raise<EOL><DEDENT>return svg<EOL>
Call dot command, and provide helpful error message if we
       cannot find it.