pickle up errors from build environments

[Sergio Schvezov]

If an error occurs while in the context of a build VM, we have no way for the user to see the error.

[Claudio Matsuoka (cmatsuoka)]

Upon facing an unhandled exception, snapcraft:

a) generates traceback data and stores it in the filesystem, and
b) tells the user where the file is (if running on an interactive terminal).

However, if running inside a build provider, the filesystem location is not accessible from the host snapcraft instance. To give user access to error details, we need to retrieve the traceback file from the managed provider. The proposed fix addresses this issue by splitting items (a) and (b) above between the outer and inner instances of snapcraft. Inner instance stores error data in a known location, which is retrieved by outer instance before proceeding to step (b). Outer snapcraft is then able to store error data on the host filesystem and tell the user where it is.

If running non-interactively, trace data is printed to stdout instead of stored in a file. This is handled directly by the inner instance.

[Claudio Matsuoka (cmatsuoka)]

Amending previous comment: for logic uniformity, all trace data is handled by the outer instance, even when non-interactive.

The following scenarios should be covered:
- with build provider/without build provider (legacy mode)
- interactive/non-interactive

[Claudio Matsuoka (cmatsuoka)]

(Legacy mode is handled in a completely separated way so it shouldn't have any interaction with this fix.)

[Claudio Matsuoka (cmatsuoka)]

The fix for this bug also fixes bug #1814429.