Add more context to errors

[d-e-s-o]

With df959af we have a custom error type that allows for additional context to be provided by the library (e.g., on top of system errors such as ENOENT). We should make use of this capability to improve the quality of error messages in general. Basically, when there is a reason to believe that a low-level error is not particularly useful without context, add some via ErrorExt::context or ErrorExt::with_context. Here is an example:

blazesym/src/dwarf/units.rs

Lines 89 to 94 in cd6456c

	let dw_unit = sections.unit(header).with_context(|| {
	format!(
	"failed to retrieve DWARF unit for unit header @ {}",
	format_offset(header.offset())
	)
	})?;

On this path it would probably make sense to add further context higher up the stack so that users know exactly which DWARF file the error occurred on.

Among other things, doing so could help with error reports such as:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error: No such file or directory (os error 2)', benches/symbolize.rs:70:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This task is a good opportunity to become familiar with the code base and it can be done in stages (every improvement helps).

[danielocfb]

I added some more context over time, whenever I felt it was not sufficient. Probably does no longer make sense to have a dedicated task open.