Misleading error on missing Range bounds

[kcpevey]

Setting Range bounds=None (or not providing them) results in a misleading error:

float_range = param.Range(default=(0.0, 1.57), bounds=None)

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-8-ede5c729d901> in <module>()
      6     colormap = param.ObjectSelector(default='rainbow', objects=['rainbow', 'bgy', 'bgyw', 'blues', 'coolwarm', 'gray,'])
      7 opts = plot_options(name='')
----> 8 parambokeh.Widgets(opts)

C:\ProgramData\Anaconda3\envs\earthsimAUG\lib\site-packages\param\parameterized.py in __new__(class_, *args, **params)
   2047         inst = class_.instance()
   2048         inst.param._set_name(class_.__name__)
-> 2049         return inst.__call__(*args,**params)
   2050 
   2051     def __call__(self,*args,**kw):

C:\ProgramData\Anaconda3\envs\earthsimAUG\lib\site-packages\parambokeh\__init__.py in __call__(self, parameterized, doc, plots, **params)
    232 
    233         # Initialize widgets and populate container
--> 234         widgets, views = self.widgets()
    235         plots = views + plots
    236         widget_box.children = widgets

C:\ProgramData\Anaconda3\envs\earthsimAUG\lib\site-packages\parambokeh\__init__.py in widgets(self)
    495 
    496         if self.p.show_labels:
--> 497             widgets += [self.widget(pname) for pname in ordered_params]
    498         else:
    499             widgets += [self.widget(pname) for pname in ordered_params]

C:\ProgramData\Anaconda3\envs\earthsimAUG\lib\site-packages\parambokeh\__init__.py in <listcomp>(.0)
    495 
    496         if self.p.show_labels:
--> 497             widgets += [self.widget(pname) for pname in ordered_params]
    498         else:
    499             widgets += [self.widget(pname) for pname in ordered_params]

C:\ProgramData\Anaconda3\envs\earthsimAUG\lib\site-packages\parambokeh\__init__.py in widget(self, param_name)
    459         """Get widget for param_name"""
    460         if param_name not in self._widgets:
--> 461             self._widgets[param_name] = self._make_widget(param_name)
    462         return self._widgets[param_name]
    463 

C:\ProgramData\Anaconda3\envs\earthsimAUG\lib\site-packages\parambokeh\__init__.py in _make_widget(self, p_name)
    403             kw['start'], kw['end'] = p_obj.get_soft_bounds()
    404 
--> 405         w = widget_class(**kw)
    406 
    407         if hasattr(p_obj, 'callbacks') and value is not None:

C:\ProgramData\Anaconda3\envs\earthsimAUG\lib\site-packages\parambokeh\widgets.py in RangeWidget(*args, **kw)
     92     if isinstance(kw['start'], int) and isinstance(kw['end'], int):
     93         kw['step'] = 1
---> 94     return RangeSlider(*args, **kw)
     95 
     96 def PlotWidget(*args, **kw):

C:\ProgramData\Anaconda3\envs\earthsimAUG\lib\site-packages\bokeh\models\widgets\sliders.py in __init__(self, **kwargs)
     22         if 'start' in kwargs and 'end' in kwargs:
     23             if kwargs['start'] == kwargs['end']:
---> 24                 raise ValueError("Slider 'start' and 'end' cannot be equal.")
     25         super(Widget, self).__init__(**kwargs)
     26 

ValueError: Slider 'start' and 'end' cannot be equal.