Add methods to construct errors from Rust strings.

This is more convenient in most cases than a CStr, and will let callers
stop duplicating the code to convert it.

Bug: 234019127
Test: atest compos_key_tests MicrodroidHostTestCases MicrodroidTestApp libbinder_rs-internal_test
Change-Id: Idc6684f22f33d4ff79d2326499d089dc85e213ea