Don't exit when sourcemaps do not exist

[robbertvancaem]

Based on this ticket

Currently, in this line we copy the sourcemaps to be included in the tgz. This can lead to a big file, which can exceed the upload limit of 10MB. We could consider two ways of handling this - one not excluding the other;

1. Increase the upload limit to for example 15MB (outside of the scope of this project). This allows for bigger apps to still have sourcemaps included, which can be handy or even necessary during development and testing on actual devices.
2. Do not exit/abort the upload action when the sourcemaps are not present. The Lightning-CLI has support for the LNG_BUILD_SOURCEMAP argument, we should probably check if this is set to anything other than true. If that's the case, the files should be allowed to not be there (causing the upload not to fail). If it's set to true, it can be expected that they ARE there, so then it makes sense if the upload/pack action fails

CC: @Thomvl