Handle errors when opening files

[Juan Pablo Caram]

Parsing errors when opening the wrong file type and similar.

[Juan Pablo Caram]

Example: Attempt to open the .dri file generated by Eagle. It is not an Excellon file so it should fail:

[ERROR] could not convert string to float: 
[ERROR] could not convert string to float: 
[ERROR] <traceback object at 0x4515878>
[ERROR] <traceback object at 0x4515878>
Traceback (most recent call last):
  File "/home/jpcaram/flatcam/FlatCAMWorker.py", line 32, in do_worker_task
    task['fcn'](*task['params'])
  File "/home/jpcaram/flatcam/FlatCAMApp.py", line 1648, in open_excellon
    self.new_object("excellon", name, obj_init)
  File "/home/jpcaram/flatcam/FlatCAMApp.py", line 778, in new_object
    initialize(obj, self)
  File "/home/jpcaram/flatcam/FlatCAMApp.py", line 1628, in obj_init
    excellon_obj.parse_file(filename)
  File "/home/jpcaram/flatcam/camlib.py", line 2184, in parse_file
    self.parse_lines(estr)
  File "/home/jpcaram/flatcam/camlib.py", line 2251, in parse_lines
    x = self.parse_number(match.group(1))
  File "/home/jpcaram/flatcam/camlib.py", line 2364, in parse_number
    return float(number_str) / 10000
ValueError: could not convert string to float:

[Juan Pablo Caram]

I just tried opening a .py as a Gerber. The object was created without errors. Errors were thrown (shown in the terminal) when trying to plot though. Nothing interesting in the GUI whatsoever. Will implement only some very simple checks and handlers for 8.5 and this issue will remain open.