Raise better exception when too many fields added to writer

[mwtoews]

While accidentally writing too many fields to a new shapefile, I get a cryptic error message raised by struct.pack:

error: ushort format requires 0 <= number <= USHRT_MAX

Here are the relevant bits of debugging through ipython, using pyshp version 1.2.12:

C:\ProgramData\Anaconda2\lib\site-packages\shapefile.pyc in save(self, target, shp, shx, dbf)
   1146             self.saveShx(target)
   1147             self.shx.close()
-> 1148             self.saveDbf(target)
   1149             self.dbf.close()
   1150             if generated:

C:\ProgramData\Anaconda2\lib\site-packages\shapefile.pyc in saveDbf(self, target)
   1117             target = os.path.splitext(target)[0] + '.dbf'
   1118         self.dbf = self.__getFileObj(target)
-> 1119         self.__dbfHeader()
   1120         self.__dbfRecords()
   1121

C:\ProgramData\Anaconda2\lib\site-packages\shapefile.pyc in __dbfHeader(self)
    803         recordLength = sum([int(field[2]) for field in self.fields]) + 1
    804         header = pack('<BBBBLHH20x', version, year, month, day, numRecs,
--> 805                 headerLength, recordLength)
    806         f.write(header)
    807         # Field descriptors

I did something equivalent to:

for n in range(1000):
    w.field("field{0}".format(n), "C")

(I can't recall exactly how many, except that it was certainly beyond USHRT_MAX)

Could we have a better exception raised somewhere? Something like ValueError("number of fields ({0}) exceeds the maximum 256".format(headerLength), or whatever is the correct DBF limit. Or possibly, raise an exception when w.field(name, ...) has been called too many times.