Previously Error() was giving "0 errors", but "" seems to be a better
choice.

Anyway proper usage of Errorv is to call .Err() when handing result to
some where so this way empty-vector case becomes nil.

- various .Append*() are functions to add errors to error vector.
- .Err() returns error in canonical form accumulated in error vector -
  e.g. nil when error vector is empty.

Name for error vector: ev -> errv

This might be frequently needed when running a goroutine - to catch
errors at top level and then convey they to other places as regular
errors.

Fix this thinko in Addcallingcontext description.

In current Error.Error() only function names are printed but with actual
frames there in error context clients will be probably able to access
this information theirselves.

Because not only go.git wants to increase verbosity on `-v -v ...`.

Code taken from go.git ec80737bdf.

No many details since they can be all viewed e.g. with help of godoc.
Just a top-level pointer.

Move them here to go123 like in previous patches. Taken from git-backup
0be1f647.

[1] https://lab.nexedi.com/kirr/git-backup

Pristine import + package/copyright clauses. Taken from git-backup 3aedc246.

[1] https://lab.nexedi.com/kirr/git-backup

Export functions that client would use and adjust their names taking
into account that now there is a package prefix. So e.g.

	raise		-> exc.Raise
	errcatch	-> exc.Catch
	erronunwind	-> exc.Onunwind
	...

It is completely orthogonal to exception handling.

It is

	xruntime.Traceback()

Usage would be:

	myname.Func(),	and
	myname.Pkg()

This is pristine import. Files are taken as-is from git-backup 3ba6cf73.
Will adapt them to live in separate package in the next commit.

[1] https://lab.nexedi.com/kirr/git-backup

This will be a place to keep handy go bits shared across projects.