Review all exception throws and demote them to asserts where appropriate

[xezon]

Throwing exceptions for non critical things is not good. It stops the runtime unnecessarily. It would be better to raise asserts and return the offending code paths. This gives the runtime the chance to just move on. This is better for the user when the error is not catastrophic.

An example of an unnecessary throw is

• [GEN][ZH] Demote throw to assert in AsciiString::format_va, UnicodeString::format_va #835