Add type checking for fixed column widths

[axoking]

When you accidentally use a string with a number as a table column width (like "690" instead of 690), pdfmake used to ignore that and then later throwed an error when trying to calculate something with that wrong length.

It used to look like this:

Error: unsupported number: 45690939.433593759012.087890625

Now, when evaluating the column widths, pdfmake checks if the widths are really numbers, and if not, it throws an error message like this:

Error: Invalid column width: 690 (width must be a number, "*" or "auto")