Raise Python exceptions as Lisp (pyerror) errors and guard numpy import

[plandes]

The big change with this pull request is to raise a Lisp pyerror when a Python error is raised. It does this by wrapping all evaluated code in a try catch that looks something like:

try:
  _ = <inserted code>
except Exception as e:
  _ = ExecutionFailure(e)

Both the stack trace and the error are retained and available in the (modified) pyerror. Example:

(handler-case
    (raw-pyexec "raise ValueError('Some error')")
  (pyerror (e)
    (format t "Caught: <~a>~%Message: <~a>~%Exception class: <~a>~%"
	    e
	    (slot-value e 'py4cl2-cffi::exception-message)
	    (slot-value e 'py4cl2-cffi::exception-type))))

produces:

Caught: <Python raised an exception:
Traceback (most recent call last):
  File "<string>", line 3, in <module>
ValueError: Some error
>
Message: <Some error>
Exception class: <ValueError>

For some unknown reason, I was not able to merge in the changes you made last night so I recreated my fork for this pull request (in case you were wondering or if there's any other git weirdness).