Add check for invalid formulas

[khsu2000]

When excel files have invalid formulas/data, this can cause xlsx2csv to produce an error like the following below (traceback truncated to only include the xlsx2csv errors):

Traceback (most recent call last):
  File "/shared/ceph/homes/khsu/data_warehouse/integration/penv/lib/python3.7/site-packages/xlsx2csv.py", line 860, in handleCharData
    self.data = ("%f" % (float(self.data))).rstrip('0').rstrip('.')
ValueError: could not convert string to float: '#N/A'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
...
  File "/shared/ceph/homes/khsu/data_warehouse/tools/src/preproc/read.py", line 155, in read_excel
    xlsx2csv.Xlsx2csv(infile).convert(outfile, sheetid=sheet_id)
  File "/shared/ceph/homes/khsu/data_warehouse/integration/penv/lib/python3.7/site-packages/xlsx2csv.py", line 235, in convert
    self._convert(sheetid, outfile)
  File "/shared/ceph/homes/khsu/data_warehouse/integration/penv/lib/python3.7/site-packages/xlsx2csv.py", line 353, in _convert
    sheet.to_csv(writer)
  File "/shared/ceph/homes/khsu/data_warehouse/integration/penv/lib/python3.7/site-packages/xlsx2csv.py", line 782, in to_csv
    self.parser.ParseFile(self.filehandle)
  File "/home/ebyrne/rpmbuild/BUILD/Python-3.7.4/Modules/pyexpat.c", line 282, in CharacterData
  File "/shared/ceph/homes/khsu/data_warehouse/integration/penv/lib/python3.7/site-packages/xlsx2csv.py", line 874, in handleCharData
    raise XlsxValueError("Error: potential invalid date format.")
xlsx2csv.XlsxValueError: Error: potential invalid date format.

This occurs with both #N/A and #VALUE!, or when excel files contain invalid formulas. Because xlsx2csv doesn't know how to handle these, this breaks the conversion. This PR aims to fix these issues by removing the faulty data instead of trying to force a conversion onto it when an invalid formula is encountered.