Fix error handling during type autodetection in create mode

[lxp]

When specifying an unknown filename without type in create mode the exception is not handled correctly:

$ cfv -C -f asdf
Traceback (most recent call last):
  File "/persist/cfv/test/cfv", line 9, in <module>
    cfv.common.main()
    ~~~~~~~~~~~~~~~^^
  File "/persist/cfv/lib/cfv/common.py", line 2314, in main
    raise CFVValueError('specify a filetype with -t, or use standard extension')
cfv.exceptions.CFVValueError: specify a filetype with -t, or use standard extension

This fix handles and displays the error correctly:

$ cfv -C -f asdf
cfv: specify a filetype with -t, or use standard extension

[sarsolot]

Good catch, had it on my TODO as well. One suggestion - instead of replacing the raise with view.perror(...) + sys.exit(1), consider wrapping the post parse execution phase in its own try/except CFVValueError and using the same cfv: ... handler there. That keeps the raise in place and handles these runtime CFVValueErrors in one consistent place.