Insert AssertionErrors in "impossible" code branches

[OLEGSHA]

Throughout the project there are multiple instances of code branches, usually catch blocks, which are commented as unreachable. For enhanced robustness, throw new AssertionError("Unreachable code"); should be placed in such places to help identify logic errors.

Code locations of interest are currently marked with comments such as "Impossible" or "X can't happen". A thorough search should be conducted in the entire codebase for these occurrences.

Example

try {
	ChunkIO.save(chunk, this.data.getWriter(), IOContext.COMMS);
} catch (IOException e) {
	// Impossible
}