fixed missing SolvingError import
fixed missing SolvingError import

>>> from pycoin.tx.pay_to import ScriptUnknown
>>> script = ScriptUnknown('abc')
>>> script.solve()
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "build/bdist.linux-x86_64/egg/pycoin/tx/pay_to/ScriptUnknown.py", line 16, in solve
    raise SolvingError("unknown script type")
NameError: global name 'SolvingError' is not defined

Imported inside ScriptUnknown.solve() method due to possible infinite import loop

devrandom/pycoin,zsulocal/pycoin,gitonio/pycoin,antiface/pycoin,tomholub/pycoin,shayanb/pycoin,thirdkey-solutions/pycoin,gitonio/pycoin,pycoin/pycoin,XertroV/pycoin,devrandom/pycoin,richardkiss/pycoin,mperklin/pycoin,Pan0ram1x/pycoin,moocowmoo/pycoin,ptcrypto/pycoin,Magicking/pycoin,zsulocal/pycoin,tomholub/pycoin,shayanb/pycoin,mperklin/pycoin,XertroV/pycoin,pycoin/pycoin,shivaenigma/pycoin,richardkiss/pycoin,ptcrypto/pycoin,antiface/pycoin,Pan0ram1x/pycoin,moocowmoo/pycoin,shivaenigma/pycoin,thirdkey-solutions/pycoin,Magicking/pycoin