[Config] Adding the previous exception message into the FileLoaderLoadException so it's more easily seen

In the framework if you, for example, make a syntax error in YAML, then the true, clear message (e.g. Unable to parse at line 5 (near "framework") is nested, and harder to visually see. This includes that message in the main exception so that users can debug more easily.

Before: Cannot import resource "/path/to/app/config/config.yml" from "/path/to/app/config/config_dev.yml".

After: Cannot import resource "/path/to/app/config/config.yml" from "/path/to/app/config/config_dev.yml". (Unable to parse at line 5 (near "framework").)
merged branch weaverryan/resource-loader-show-previous-exception (PR #8089)

This PR was merged into the 2.3 branch.

Discussion
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[Config] Including exception message to clarify the underlying issue

Hi guys!

In the framework if you, for example, make a syntax error in YAML, then the true, clear message (e.g. Unable to parse at line 5 (near "framework") is nested, and harder to visually see. This includes that message in the main exception so that users can debug more easily.

I see this quite a bit, it's a choke point for newcomers :).

Before: Cannot import resource "/path/to/app/config/config.yml" from "/path/to/app/config/config_dev.yml".

After: Cannot import resource "/path/to/app/config/config.yml" from "/path/to/app/config/config_dev.yml". (Unable to parse at line 5 (near "framework").)

Corrections and comments warmly appreciated.

Thanks!

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | --
| License       | MIT
| Doc PR        | n/a

Commits
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435012f [Config] Adding the previous exception message into the FileLoaderLoadException so it's more easily seen