Improve "file not found" error messages

[szokeasaurusrex]

Currently, we have some "file not found" error messages, which are not super helpful, because they don't mention the file path that Sentry CLI was looking for.

We should add the file path to these error messages, to make them more actionable for users.

Ideally, whenever we attempt to open a file, we would add context to the error message which includes the name of the file we are trying to open, and perhaps some additional information about the context in which we are opening the file. For instance, if we are trying to open a sourcemap file at /path/to/sourcemap.map, we could make the error message something like the following:

Error while opening sourcemap `/path/to/sourcemap.map`: File not found

Exact format could look different, but we should aim to include this information in the error messages to make them more user-friendly.