Merge pull request #2675 from lambdageek/dev/monoerror-mono_string_intern

Use MonoError in mono_string_{is_interned,intern}

Also mark mono_string_intern external only.  Runtime should use
mono_string_intern_checked.

diff --cc mono/metadata/object-internals.h
Simple merge

diff --cc mono/metadata/object.c
Simple merge

diff --cc mono/metadata/object.h
Simple merge

diff --cc mono/metadata/reflection.c
Simple merge