style: formalize exception policy on the data path. (envoyproxy#4793)

Signed-off-by: Harvey Tuch <[email protected]>

STYLE.md

@@ -17,6 +17,18 @@
 
 * Exceptions are allowed and encouraged where appropriate. When using exceptions, do not add
   additional error handing that cannot possibly happen in the case an exception is thrown.
+* Do use exceptions for:
+  - Configuration ingestion error handling. Invalid configurations (dynamic and
+    static) should throw meaningful `EnvoyException`s, the configuration
+    ingestion code will catch these.
+  - Constructor failure.
+  - Error handling in deep call stacks, where exceptions provide material
+    improvements to code complexity and readability.
+* Apply caution when using exceptions on the data path for general purpose error
+  handling. Exceptions are not caught on the data path and they should not be
+  used for simple error handling, e.g. with shallow call stacks, where explicit
+  error handling provides a more readable and easier to reason about
+  implementation.
 * References are always preferred over pointers when the reference cannot be null. This
   includes both const and non-const references.
 * Function names should all use camel case starting with a lower case letter (e.g., `doFoo()`).