When failing to load assembly dependency, print better error message

I'm running the @core branch of azure-functions-core-tools.

One of my dependencies was not in my source folder (due to x86 vs x64), and I got this error from the CLI:

[9/7/2017 1:51:06 AM] ScriptHost initialization failed
[9/7/2017 1:51:06 AM] System.Private.CoreLib: Could not load file or assembly 'MosaicMakerFunction, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.

Start by reproducing the assembly-load failure on the @core branch through the CLI and follow the ScriptHost initialization path. Inspect how the CLI surfaces the System.Private.CoreLib load error. Done means a missing or incompatible assembly dependency produces a more actionable error message than the current generic failure.

Written by the indexing model from the issue text.