Give more helpful error messages when files are missing

The current error message talks a lot about $srctree, but $srctree is
seldom the culprit in practice. More common is
'source "$(SOME_ENV_VAR)/foo"`, where SOME_ENV_VAR hasn't been set.

Include the complete 'source ...' line for missing Kconfig files, and
mention unset environment variables as a hint. Only mention $srctree
briefly.

Also shorten the message when a .config can't be a found a bit. This
message would usually only be seen when working directly with the
library.

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