Requesting a nonexistent file generates a Length prefix missing error on Veracruz-Client

[gbryant-arm]

Describe the bug
Requesting a nonexistent file generates a Length prefix missing error on Veracruz-Client.
This is because read_file_by_absolute_path() fails in read_file(), which ultimately results in the RM failing to send TLS data and sending an empty HTTP response.
@egrimley-arm 's current work on removing HTTP between VC-Client and RM might have an impact on that.

To Reproduce
Request a file that doesn't exist:

<client> --result /nonexisting_file=- --identity <...> --key <...>

Expected behaviour
The runtime manager should always prefix the messages it sends with the protocol buffer's length.

[egrimley-arm]

I'm not sure that the person or program that invokes vc-client needs to know about the details of whether protocol buffers are prefixed with lengths. The main thing is that vc-client writes nothing to stdout and exits with a non-zero status code, right? It might be doing that correctly after #552 is merged, but we should check.

[gbryant-arm]

I agree that the protocol buffer handling should be transparent to the invoker.
Just saying a FS-related error shouldn't propagate to the TLS layer.
Additionally, for security reasons, we should try to minimise the number of errors the RM can return.
Sure, let's wait for #552 to be merged 👍

[egrimley-arm]

PR #584 might deal with this.