Clarify length of windows

I found that the window can only be a single block, i.e., I cannot have
multiple windows.

I added a description about this on the documentation and make it sure
in the code.

It is desirable to have multiple windows, then one can measure, for
example, just the wings of the spectral line.
Modify how to catch the error

I changed the current method of catching the error on the
Synfit.sample_params function to a try...except code.

I also wrote a test to see if the error is been throwed correctly.
Change how to catch the error on observation file

Instead of using a if...rais statement, I changed to a try...except
statement.