[Ada] Restore context on failure in loading of renamed child unit

[Pierre-Marie de Rodat]

When loading of renamed child unit failed, we didn't properly restore
the value of a global Parsing_Main_Extended_Source variable.

This is primarily a cleanup change; behaviour is not affected (perhaps
except for errors reported on complicated code that is illegal anyway).

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* lib-load.adb (Load): Replace early return with goto to properly
	restore context on failure.
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