Separation of exceptions

Moved exceptions into their own files for easier readability.

Tests pass, coverage has increased from 86% to 88%:

```
Name                                                Stmts   Miss  Cover   Missing
---------------------------------------------------------------------------------
twilio                                                  3      0   100%   
twilio.compat                                          10      2    80%   18-20
twilio.exceptions                                       4      0   100%   
twilio.jwt                                             53      7    87%   18-19, 62-63, 70-71, 78
twilio.rest                                            95      8    92%   47, 124-125, 160, 162, 166, 172-173
twilio.rest.exceptions                                 28     16    43%   33-69
twilio.rest.resources                                  23      0   100%   
twilio.rest.resources.accounts                         43     10    77%   58, 64, 70, 76, 93, 105, 111, 117, 123, 131
twilio.rest.resources.applications                     17      2    88%   11, 32
twilio.rest.resources.base                            185     12    94%   22-26, 41-43, 251, 263, 379, 413, 431
twilio.rest.resources.caller_ids                       22      9    59%   11, 17, 31, 38, 44, 69-72
twilio.rest.resources.calls                            50      7    86%   29-30, 36-37, 45-46, 146
twilio.rest.resources.conferences                      39      9    77%   13, 19, 25, 41, 47, 53, 59, 65, 72
twilio.rest.resources.connect_apps                     22      0   100%   
twilio.rest.resources.connection                       13      0   100%   
twilio.rest.resources.imports                          22     12    45%   4-5, 10-14, 25-29
twilio.rest.resources.media                            22      2    91%   43, 86
twilio.rest.resources.messages                         21      1    95%   139
twilio.rest.resources.notifications                    15      1    93%   11
twilio.rest.resources.phone_numbers                    92      4    96%   237, 260, 295, 317
twilio.rest.resources.queues                           30      2    93%   146, 154
twilio.rest.resources.recordings                       20      0   100%   
twilio.rest.resources.sandboxes                        15      5    67%   13-14, 24, 30-32
twilio.rest.resources.sip                              29      8    72%   32-33, 40-41, 48-52, 59-63
twilio.rest.resources.sip.credential_lists             34     12    65%   34, 40, 54-55, 62, 69, 102, 108, 121-122, 130, 138
twilio.rest.resources.sip.domains                      42     14    67%   9, 23-24, 31, 39, 53-54, 61, 135, 141, 172-173, 183, 191
twilio.rest.resources.sip.ip_access_control_lists      35     13    63%   35, 41, 55-57, 64, 71, 102, 108, 121-122, 131, 139
twilio.rest.resources.sms_messages                     40      8    80%   8, 47-53, 166-167
twilio.rest.resources.transcriptions                   11      0   100%   
twilio.rest.resources.usage                            69      3    96%   12, 70, 112
twilio.rest.resources.util                             61      2    97%   56, 132
twilio.twiml                                          243     13    95%   123, 128, 158, 163, 173, 183, 186, 201, 393, 395, 398, 411, 425
twilio.util                                            70      2    97%   59, 164
twilio.version                                          2      0   100%   
---------------------------------------------------------------------------------
TOTAL                                                1480    184    88%   
----------------------------------------------------------------------
Ran 241 tests in 0.197s

OK
```