Improve an error message.

See #1362.

cabal-install/Distribution/Client/Install.hs

@@ -173,7 +173,7 @@ install verbosity packageDBs repos comp platform conf useSandbox mSandboxPkgInfo
   userTargets0 = do
 
     installContext <- makeInstallContext verbosity args (Just userTargets0)
-    installPlan    <- foldProgress logMsg die return =<<
+    installPlan    <- foldProgress logMsg die' return =<<
                       makeInstallPlan verbosity args installContext
 
     processInstallPlan verbosity args installContext installPlan
@@ -183,6 +183,14 @@ install verbosity packageDBs repos comp platform conf useSandbox mSandboxPkgInfo
             globalFlags, configFlags, configExFlags, installFlags,
             haddockFlags)
 
+    die' message = die (message ++ if isUseSandbox useSandbox
+                                   then installFailedInSandbox else [])
+    -- TODO: use a better error message, remove duplication.
+    installFailedInSandbox =
+      "Note: when using a sandbox, all packages are required to have \
+      \consistent dependencies. \
+      \Try reinstalling/unregistering the offending packages or \
+      \recreating the sandbox."
     logMsg message rest = debugNoWrap verbosity message >> rest
 
 -- TODO: Make InstallContext a proper datatype with documented fields.

cabal-install/Distribution/Client/Sandbox.hs

@@ -542,7 +542,7 @@ reinstallAddSourceDeps verbosity configFlags' configExFlags
       -- might want to use some lower-level features this in the future.
       withSandboxBinDirOnSearchPath sandboxDir $ do
         installContext <- makeInstallContext verbosity args Nothing
-        installPlan    <- foldProgress logMsg die return =<<
+        installPlan    <- foldProgress logMsg die' return =<<
                           makeInstallPlan verbosity args installContext
 
         processInstallPlan verbosity args installContext installPlan
@@ -551,6 +551,13 @@ reinstallAddSourceDeps verbosity configFlags' configExFlags
   readIORef retVal
 
     where
+      die' message = die (message ++ installFailedInSandbox)
+      -- TODO: use a better error message, remove duplication.
+      installFailedInSandbox =
+        "Note: when using a sandbox, all packages are required to have \
+        \consistent dependencies. \
+        \Try reinstalling/unregistering the offending packages or \
+        \recreating the sandbox."
       logMsg message rest = debugNoWrap verbosity message >> rest
 
       topHandler' = topHandlerWith $ \_ -> do