fixed error in LibGit2.merge! when a branch does not have a remote (#15937)

* fixed merge error when branch does not have remote

fixed error in an exception message creation when exception isn't GitError
(Found it while looked at #14616)

testing LibGit2.merge! exceptions

removed changes in original message

fixed #13991, rethrowing uknown errors

defer exceptions until update end

moved statement to avoid contamination of backtrace

* fixed similar error in `rebase!`, fixed typos

* Separate internal API and Pkg UI commands by using `PkgError`
`PkgError` becomes an information error that would keep underlying exceptions
`CapturedException` should have supertype `Exception`

* Put off `Cache.prefetch` exceptions in `Pkg.update`

* enable `fetch` exceptions, nice `PkgError` output

* fix possible undefined behavior (ref pull/14146)

* fixed indentations

Author: wildart

base/pkg.jl

@@ -11,6 +11,23 @@ const META_BRANCH = "metadata-v2"
 
 type PkgError <: Exception
     msg::AbstractString
+    ex::Nullable{Exception}
+end
+PkgError(msg::AbstractString) = PkgError(msg, Nullable{Exception}())
+function Base.showerror(io::IO, pkgerr::PkgError)
+    print(io, pkgerr.msg)
+    if !isnull(pkgerr.ex)
+        pkgex = get(pkgerr.ex)
+        if isa(pkgex, CompositeException)
+            for cex in pkgex
+                print(io, "\n=> ")
+                showerror(io, cex)
+            end
+        else
+            print(io, "\n")
+            showerror(io, pkgex)
+        end
+    end
 end
 
 for file in split("dir types reqs cache read query resolve write entry git")