Better Error Messages for Text Format Parsing

[reventlov]

UpdateFromText() just returns a bool indicating success/failure. There is currently no way to find out where parsing failed, which makes it difficult to debug manually-written text format structs.

[reventlov]

The implementation of this is probably to update the C++ UpdateFromTextStream() methods to take an optional output parameter, or to build a parallel API like UpdateFromTextStreamOrReturnError(). I haven't given a lot of thought to the best way to actually return the error(s). Right now, the update bails out if there is any error, but I can imagine having an allow_partial_update option similar to the allow_partial_output option for text output, so ideally the API would allow multiple errors to be returned.

A (maybe) comprehensive list of the places that are are likely to need to be changed:

• UpdateFromTextStream() on generated structure (struct and bits) views; tests in text_format_test.cc, nested_structure_test.cc, bits_test.cc, anonymous_bits_test.cc, uint_sizes_test.cc, and complex_structure_test.cc
• UpdateFromTextStream() on virtual fields; tests in virtual_field_test.cc
• UpdateFromTextStream() on arrays, which really just calls ReadArrayFromTextStream(); tests in emboss_array_view_test.cc
• UpdateFromTextStream() on enum; tests in emboss_enum_view_test.cc
• UpdateFromTextStream() on built-in types: Flag, UInt, Int, Bcd, and Float; tests are all in emboss_prelude_test.cc
• The UpdateFromText convenience function should also get an optional parameter or alternate API.
• There are references to UpdateFromText or UpdateFromTextStream in text-format.md, cpp-guide.md, and cpp-reference.md.