Unify error handling: remove panics, esp. file I/O

[mLaHire]

Remove calls to panic!(), in src/lib.rs load_binary.rs and linker

Construct a graceful hierarchy of error propagation, and graceful retries, e.g., around file extensions; where file does not have an extension, first try file, and if it does not exist, try the appropriate extension then return an error(?).