capitalise error message

Co-authored-by: Eelco Dolstra <[email protected]>

Author: fricklerhandwerk

Diff for: src/libstore/build/derivation-goal.cc

@@ -762,10 +762,10 @@ void DerivationGoal::tryToBuild()
 void DerivationGoal::tryLocalBuild() {
     throw Error(
         R"(
-        unable to build with a primary store that isn't a local store;
+        Unable to build with a primary store that isn't a local store;
         either pass a different '--store' or enable remote builds.
 
-        for more information check 'man nix.conf' and search for '/machines'
+        For more information check 'man nix.conf' and search for '/machines'.
         )"
     );
 }