Use relative paths from workspace root in wasm_error! messages

[mattyg]

Currently the wasm_error! macro uses file!() macro to include the path to file where the error occured. Because this is generated at compile time, it shows the path from the machine where the wasm was compiled.

This can lead to confusing error messages as consumers will see a filesystem they don't recognize (i.e. (/home/matt/Projects/Holochain/...). They may not be sure if something is not working properly, or maybe an incorrect path is the source of their error. This came up while supporting development of Volla Messages.

We should investigate if we can use relative paths from the workspace root

[mattyg]

It does look like WasmError file paths are relative (see #140). Will need to investigate further.