preserve last exception when things are going wrong

CircuitPython 8 supports exception chaining, so that the original
problem can be shown. Here's how it looks on desktop python3:
```
Traceback (most recent call last):
  File "/home/jepler/src/bundle/libraries/helpers/requests/adafruit_requests.py", line 527, in _get_socket
    sock.connect((connect_host, port))
ConnectionRefusedError: [Errno 111] Connection refused

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/jepler/src/bundle/libraries/helpers/requests/adafruit_requests.py", line 721, in get
    return self.request("GET", url, **kw)
  File "/home/jepler/src/bundle/libraries/helpers/requests/adafruit_requests.py", line 661, in request
    socket = self._get_socket(host, port, proto, timeout=timeout)
  File "/home/jepler/src/bundle/libraries/helpers/requests/adafruit_requests.py", line 508, in _get_socket
    raise RuntimeError("Sending request failed") from last_exc
RuntimeError: Sending request failed
```

Author: jepler

adafruit_requests.py

@@ -499,19 +499,22 @@ def _get_socket(
         )[0]
         retry_count = 0
         sock = None
+        last_exc = None
         while retry_count < 5 and sock is None:
             if retry_count > 0:
                 if any(self._socket_free.items()):
                     self._free_sockets()
                 else:
-                    raise RuntimeError("Sending request failed")
+                    raise RuntimeError("Sending request failed") from last_exc
             retry_count += 1
 
             try:
                 sock = self._socket_pool.socket(addr_info[0], addr_info[1])
-            except OSError:
+            except OSError as exc:
+                last_exc = exc
                 continue
-            except RuntimeError:
+            except RuntimeError as exc:
+                last_exc = exc
                 continue
 
             connect_host = addr_info[-1][0]
@@ -522,15 +525,17 @@ def _get_socket(
 
             try:
                 sock.connect((connect_host, port))
-            except MemoryError:
+            except MemoryError as exc:
+                last_exc = exc
                 sock.close()
                 sock = None
-            except OSError:
+            except OSError as exc:
+                last_exc = exc
                 sock.close()
                 sock = None
 
         if sock is None:
-            raise RuntimeError("Repeated socket failures")
+            raise RuntimeError("Repeated socket failures") from last_exc
 
         self._open_sockets[key] = sock
         self._socket_free[sock] = False
@@ -650,13 +655,15 @@ def request(
         # We may fail to send the request if the socket we got is closed already. So, try a second
         # time in that case.
         retry_count = 0
+        last_exc = None
         while retry_count < 2:
             retry_count += 1
             socket = self._get_socket(host, port, proto, timeout=timeout)
             ok = True
             try:
                 self._send_request(socket, host, method, path, headers, data, json)
-            except OSError:
+            except OSError as exc:
+                last_exc = exc
                 ok = False
             if ok:
                 # Read the H of "HTTP/1.1" to make sure the socket is alive. send can appear to work
@@ -676,7 +683,7 @@ def request(
             socket = None
 
         if not socket:
-            raise OutOfRetries("Repeated socket failures")
+            raise OutOfRetries("Repeated socket failures") from last_exc
 
         resp = Response(socket, self)  # our response
         if "location" in resp.headers and 300 <= resp.status_code <= 399: