VUnit exits with vague exception when GTKwave not available

[joshrsmith]

If using GHDL + GTKwave, VUnit will exit with a vague exception is GTKwave is not available. It would be more helpful if a check for GTKWave on the path could be done before it was run. I originally suspected that there might have been a windows path length error, which can also throw this type of exception, before I discovered that GTKwave was not properly on my path.

For example:

(venv) C:\Users\Josh\projects\vunit_workspace\vunit\examples\vhdl\uart>python run.py --gtkwave vcd
Starting tb_uart_lib.tb_uart_rx.test_tvalid_low_at_start
gtkwave C:\Users\Josh\projects\vunit_workspace\vunit\examples\vhdl\uart\vunit_out\tests\tb_uart_lib.tb_uart_rx.test_tvalid_low_at_start\ghdl\wave.vcd
simulation stopped @1ms with status 0
Traceback (most recent call last):
  File "C:\Users\Josh\projects\vunit_workspace\venv\lib\site-packages\vunit\test_runner.py", line 149, in _run_test_suite
    results = test_suite.run(output_path)
  File "C:\Users\Josh\projects\vunit_workspace\venv\lib\site-packages\vunit\test_list.py", line 88, in run
    test_ok = self._test_case.run(output_path)
  File "C:\Users\Josh\projects\vunit_workspace\venv\lib\site-packages\vunit\test_suites.py", line 55, in run
    if not self._test_bench.run(output_path, generics):
  File "C:\Users\Josh\projects\vunit_workspace\venv\lib\site-packages\vunit\test_bench.py", line 48, in run
    self._sim_config)
  File "C:\Users\Josh\projects\vunit_workspace\venv\lib\site-packages\vunit\ghdl_interface.py", line 216, in simulate
    subprocess.call(cmd)
  File "c:\python27\Lib\subprocess.py", line 522, in call
    return Popen(*popenargs, **kwargs).wait()
  File "c:\python27\Lib\subprocess.py", line 710, in __init__
    errread, errwrite)
  File "c:\python27\Lib\subprocess.py", line 958, in _execute_child
    startupinfo)
WindowsError: [Error 2] The system cannot find the file specified
fail (P=0 S=0 F=1 T=6) tb_uart_lib.tb_uart_rx.test_tvalid_low_at_start (0.3 seconds)