Rework usage of FPDF_GetLastError() and PdfiumLibraryBindings::get_pdfium_last_error().

[ajrcarey]

Indirect follow-on from #60. As pointed out by https://github.com/mara004, FPDF_GetLastError() only applies to nominated functions related to document loading, not to the Pdfium library generally. As a result, it is actually potentially harmful to use this function to check if other API calls returned an error, as it may return a false positive by re-raising an error previously already caught during document loading (since a call to FPDF_GetLastError() does not clear the error flag).

A recent update to the inline documentation clarifies that FPDF_GetLastError() should only be checked after specific functions: https://pdfium.googlesource.com/pdfium/+/e116d6ebc253640da997d10b533dd9aed6aa1c23

In light of this, rework all usage of FPDF_GetLastError() and PdfiumLibraryBindings::get_pdfium_last_error(). Consider removing get_pdfium_last_error() entirely.

[mara004]

Thanks for the clear explanation on possible false positives.
Apart from that, calling FPDF_GetLastError() in other places is also just unnecessary and may have a slightly negative performance impact.

[ajrcarey]

Removed PdfiumLibraryBindings::get_pdfium_last_error(). Reworked all usage of get_pdfium_last_error(). Restricted usage of FPDF_GetLastError() to Pdfium::pdfium_document_handle_to_result(). Confirmed all tests pass. Updated README.md.

[ajrcarey]

Scheduled for release as part of 0.8.0.