nix: workaround an issue with crate2nix, change some errors to be more clear

nix/Cargo.nix

@@ -14287,7 +14287,7 @@ rec {
           {
             name = "csv";
             packageId = "csv";
-            optional = true;
+            # optional = true;
           }
           {
             name = "directories-next";
@@ -14396,7 +14396,7 @@ rec {
           {
             name = "structopt";
             packageId = "structopt";
-            optional = true;
+            # optional = true;
           }
           {
             name = "sum_type";

nix/Cargo.nix_patches.md

@@ -0,0 +1,6 @@
+Apply these instructions after generating the `Cargo.nix` file.
+
+- Find `veloren-common` crate in `Cargo.nix`:
+	- Comment the `optional = true;` line for "structopt" and "csv" dependencies.
+	- See [this issue](https://github.com/kolloch/crate2nix/issues/129) on `crate2nix` repository for more info.
+	- Note that the suggested workaround in the issue **does not** work for us.

nix/README.md

@@ -192,6 +192,7 @@ To update `Cargo.nix` (and `crate-hashes.json`) using latest `Cargo.lock`, run:
 ```shell
 crate2nix generate -f ../Cargo.toml
 ```
+Then follow the instructions in the `Cargo.nix_patches.md` file.
 
 ### Rust toolchain
 

nix/veloren.nix

@@ -37,7 +37,7 @@ let
   prettyRev = with sourceInfo;
     if sourceInfo ? rev && sourceInfo ? lastModified
     then builtins.substring 0 8 rev + "/" + utils.dateTimeFormat lastModified
-    else throw "Need revision + lastModified to determine version";
+    else throw "Need revision + lastModified to determine pretty revision";
 
   tag = with sourceInfo;
     if sourceInfo ? tag
@@ -51,7 +51,7 @@ let
   version =
     if tag != "" then tag
     else if prettyRev != "" then prettyRev
-    else throw "Need a tag or at least revision + lastModified in order to determine version";
+    else throw "Need a tag or pretty revision in order to determine version";
 
   veloren-assets = pkgs.runCommand "makeAssetsDir" { } ''
     mkdir $out