Warn about orphaned object types

[quadrifolia]

Better error when the object type is not available because it is only used as object type within an interface. The "multiple types" error on ($runtimeType !== $this->exeContext->schema->getType($runtimeType->name)) is misleading, because the type is not available in the schema at all.

[mfn]

Shouldn't there be a test for this?

[quadrifolia]

Probably yes. I just did a quick edit within the github website. It is just a new error message (no type detected) for this problem that took me some time because the existing error message (multiple types detected) led me into the wrong direction.

[vladar]

This is useful. But this PR breaks the build (in the coding style) + we need some tests for this

[spawnia]

@quadrifolia are you still interested in pursuing this PR?

[codecov]

Codecov Report

Merging #491 (eebb225) into master (299be4f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #491   +/-   ##
=======================================
  Coverage   94.28%   94.28%           
=======================================
  Files         117      117           
  Lines        9672     9679    +7     
=======================================
+ Hits         9119     9126    +7     
  Misses        553      553           

Impacted Files	Coverage Δ
src/Executor/ReferenceExecutor.php	95.11% <100.00%> (+0.07%)	⬆️

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