[Bug] Give a proper error message when developer execute fails

[vicsn]

🐛 Bug Report

Steps to Reproduce

1. Start a snarkOS network with network 1
2. Call snarkos developer execute with any other network (e.g. the default 0)
3. It returns an empty non-descriptive error: ⚠️

Expected Behavior

The returned error message should be clear.
Please also investigate other potential errors.

[Jos Dehaes]

@vicsn @raychu86 I fixed this, but I can't assign myself