fails:Encoding::Converter#last_error returns nil when the no conversion has been attempted
fails:Encoding::Converter#last_error returns nil when the last conversion did not produce an error
fails:Encoding::Converter#last_error returns nil when #primitive_convert last returned :destination_buffer_full
fails:Encoding::Converter#last_error returns nil when #primitive_convert last returned :finished
fails:Encoding::Converter#last_error returns nil if the last conversion succeeded but the penultimate failed
fails:Encoding::Converter#last_error returns an Encoding::InvalidByteSequenceError when #primitive_convert last returned :invalid_byte_sequence
fails:Encoding::Converter#last_error returns an Encoding::UndefinedConversionError when #primitive_convert last returned :undefined_conversion
fails:Encoding::Converter#last_error returns an Encoding::InvalidByteSequenceError when #primitive_convert last returned :incomplete_input
fails:Encoding::Converter#last_error returns an Encoding::InvalidByteSequenceError when the last call to #convert produced one
fails:Encoding::Converter#last_error returns an Encoding::UndefinedConversionError when the last call to #convert produced one
