Improve build errors and remove useless grpc prefix

[tonistiigi]

Before:

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 > [st3 2/2] RUN sleep 1 && echo foobar && exit 1:
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Dockerfile:9
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   7 |     
   8 |     from alpine as st3
   9 | >>> run sleep 1 && echo foobar && exit 1
  10 |     
  11 |     from scratch
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error: failed to solve: rpc error: code = Unknown desc = executor failed running [/bin/sh -c sleep 1 && echo foobar && exit 1]: exit code: 1

After:

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 > [st3 2/2] RUN sleep 1 && echo foobar && exit 1:
#10 2.308 foobar
------
Dockerfile:9
--------------------
   7 |     
   8 |     from alpine as st3
   9 | >>> run sleep 1 && echo foobar && exit 1
  10 |     
  11 |     from scratch
--------------------
error: failed to solve: process "/bin/sh -c sleep 1 && echo foobar && exit 1" did not complete successfully: exit code: 1

Expecting tests that check error strings to fail.