Add context manager for Error collector

[phackstock]

Using a context manager for the ErrorCollector() class could help reduce boilerplate and make our code more readable.
We'd be going from:

errors = ErrorCollector()
for item in items:
    if item does not meet condition:
        errors.append(ValueError("Item does not meet condition")
    )        
if errors:
    raise ValueError(errors)

to:

with ErrorCollector():
    for item in items:
        if item does not meet condition:
            raise ValueError("Item does not meet condition")