<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def getdarkcurrent(self,chip):
 """ Return the dark current for the WFC3 UVIS detector. This value will be contained within an instrument specific keyword. Returns ------- darkcurrent: float The dark current value with **units of electrons**. """

        darkcurrent = 0.

        try:
            darkcurrent = self._image[self.scienceExt, chip].header['MEANDARK']

        except:
            msg =  "#############################################\n"
            msg += "#                                           #\n"
            msg += "# Error:                                    #\n"
            msg += "#   Cannot find the value for 'MEANDARK'    #\n"
            msg += "#   in the image header.  WFC3 input images #\n"
            msg += "#   are expected to have this header        #\n"
            msg += "#   keyword.                                #\n"
            msg += "#                                           #\n"
            msg += "# Error occured in WFC3UVISInputImage class #\n"
            msg += "#                                           #\n"
            msg += "#############################################\n"
            raise ValueError(msg)

        return darkcurrent