improve some Negative_Read/Negative_Write logic

Returns the actual error if one is set, instead of swallowing it for the
less descriptive negative error.

Also fixes a out-of-bounds slice error in `bufio.writer_write` because
it wasn't checking the returned `m`.

core/bufio/reader.odin

@@ -81,7 +81,7 @@ _reader_read_new_chunk :: proc(b: ^Reader) -> io.Error {
 	for i := b.max_consecutive_empty_reads; i > 0; i -= 1 {
 		n, err := io.read(b.rd, b.buf[b.w:])
 		if n < 0 {
-			return .Negative_Read
+			return err if err != nil else .Negative_Read
 		}
 		b.w += n
 		if err != nil {
@@ -189,7 +189,7 @@ reader_read :: proc(b: ^Reader, p: []byte) -> (n: int, err: io.Error) {
 		if len(p) >= len(b.buf) {
 			n, b.err = io.read(b.rd, p)
 			if n < 0 {
-				return 0, .Negative_Read
+				return 0, b.err if b.err != nil else .Negative_Read
 			}
 
 			if n > 0 {
@@ -202,7 +202,7 @@ reader_read :: proc(b: ^Reader, p: []byte) -> (n: int, err: io.Error) {
 		b.r, b.w = 0, 0
 		n, b.err = io.read(b.rd, b.buf)
 		if n < 0 {
-			return 0, .Negative_Read
+			return 0, b.err if b.err != nil else .Negative_Read
 		}
 		if n == 0 {
 			return 0, _reader_consume_err(b)
@@ -290,7 +290,7 @@ reader_write_to :: proc(b: ^Reader, w: io.Writer) -> (n: i64, err: io.Error) {
 	write_buf :: proc(b: ^Reader, w: io.Writer) -> (i64, io.Error) {
 		n, err := io.write(w, b.buf[b.r:b.w])
 		if n < 0 {
-			return 0, .Negative_Write
+			return 0, err if err != nil else .Negative_Write
 		}
 		b.r += n
 		return i64(n), err

core/bufio/writer.odin

@@ -95,6 +95,10 @@ writer_write :: proc(b: ^Writer, p: []byte) -> (n: int, err: io.Error) {
 		m: int
 		if writer_buffered(b) == 0 {
 			m, b.err = io.write(b.wr, p)
+			if m < 0 && b.err == nil {
+				b.err = .Negative_Write
+				break
+			}
 		} else {
 			m = copy(b.buf[b.n:], p)
 			b.n += m

core/bytes/buffer.odin

@@ -359,7 +359,7 @@ buffer_read_from :: proc(b: ^Buffer, r: io.Reader) -> (n: i64, err: io.Error) #n
 		resize(&b.buf, i)
 		m, e := io.read(r, b.buf[i:cap(b.buf)])
 		if m < 0 {
-			err = .Negative_Read
+			err = e if e != nil else .Negative_Read
 			return
 		}