Hard to debug compiler errors

[mormj]

A couple of compiler errors happen quite easily (due to user error) but can be traced to a number of things. Would be good for the compiler to output more descriptive errors.

Namely:

• missing processBulk or processOne error
• no matching function for call to ‘gr::Graph::emplaceBlock()

for example, the second error occurs when not properly instantiating a block with the correct template parameter