Show better error when standard library cannot be found.

src/nimblepkg/nimscriptsupport.nim

@@ -188,7 +188,10 @@ proc setupVM(module: PSym; scriptName: string, flags: Flags): PEvalContext =
       else:
         flags[key] = @[value]
 
-proc getNimPrefixDir(): string =
+proc isValidLibPath(lib: string): bool =
+  return fileExists(lib / "system.nim")
+
+proc getNimPrefixDir: string =
   let env = getEnv("NIM_LIB_PREFIX")
   if env != "":
     return env
@@ -221,6 +224,19 @@ proc execScript(scriptName: string, flags: Flags, options: Options): PSym =
   # Ensure the compiler can find its standard library #220.
   compiler_options.gPrefixDir = getNimPrefixDir()
 
+  # Verify that lib path points to existing stdlib.
+  compiler_options.setDefaultLibpath()
+  if not isValidLibPath(compiler_options.libpath):
+    let msg = "Nimble cannot find Nim's standard library.\nLast try in:\n  - $1" %
+              compiler_options.libpath
+    let hint = "Nimble does its best to find Nim's standard library, " &
+               "sometimes this fails. You can set the environment variable " &
+               "NIM_LIB_PREFIX to where Nim's `lib` directory is located as " &
+               "a workaround. " &
+               "See https://github.com/nim-lang/nimble#troubleshooting for " &
+               "more info."
+    raiseNimbleError(msg, hint)
+
   let pkgName = scriptName.splitFile.name
 
   # Ensure that "nimblepkg/nimscriptapi" is in the PATH.

src/nimblepkg/packageparser.nim

@@ -329,12 +329,14 @@ proc readPackageInfo(nf: NimbleFile, options: Options,
       try:
         readPackageInfoFromNims(nf, options, result)
         result.isNimScript = true
-      except NimbleError:
+      except NimbleError as exc:
+        if exc.hint.len > 0:
+          raise
         let msg = "Could not read package info file in " & nf & ";\n" &
                   "  Reading as ini file failed with: \n" &
                   "    " & iniError.msg & ".\n" &
                   "  Evaluating as NimScript file failed with: \n" &
-                  "    " & getCurrentExceptionMsg() & "."
+                  "    " & exc.msg & "."
         raise newException(NimbleError, msg)
 
   # By default specialVersion is the same as version.