Consistent behavior of exec; Improved docs around exec, staticExec, gorgeEx, etc. (#10967)

* improved docs around exec, staticExec, gorgeEx, etc.
* incorporate review comment; made behavior of exec consistent

lib/system.nim

@@ -4060,7 +4060,8 @@ proc gorge*(command: string, input = "", cache = ""): string {.
 
 proc staticExec*(command: string, input = "", cache = ""): string {.
   magic: "StaticExec".} = discard
-  ## Executes an external process at compile-time.
+  ## Executes an external process at compile-time and returns its text output
+  ## (stdout + stderr).
   ##
   ## If `input` is not an empty string, it will be passed as a standard input
   ## to the executed program.

lib/system/nimscript.nim

@@ -239,8 +239,14 @@ proc cpDir*(`from`, to: string) {.raises: [OSError].} =
     copyDir `from`, to
     checkOsError()
 
-proc exec*(command: string) =
-  ## Executes an external process.
+proc exec*(command: string) {.
+  raises: [OSError], tags: [ExecIOEffect].} =
+  ## Executes an external process. If the external process terminates with
+  ## a non-zero exit code, an OSError exception is raised.
+  ##
+  ## **Note:** If you need a version of ``exec`` that returns the exit code
+  ## and text output of the command, you can use `system.gorgeEx
+  ## <system.html#gorgeEx,string,string,string>`_.
   log "exec: " & command:
     if rawExec(command) != 0:
       raise newException(OSError, "FAILED: " & command)
@@ -248,11 +254,16 @@ proc exec*(command: string) =
 
 proc exec*(command: string, input: string, cache = "") {.
   raises: [OSError], tags: [ExecIOEffect].} =
-  ## Executes an external process.
+  ## Executes an external process. If the external process terminates with
+  ## a non-zero exit code, an OSError exception is raised.
   log "exec: " & command:
-    echo staticExec(command, input, cache)
+    let (output, exitCode) = gorgeEx(command, input, cache)
+    if exitCode != 0:
+      raise newException(OSError, "FAILED: " & command)
+    echo output
 
-proc selfExec*(command: string) =
+proc selfExec*(command: string) {.
+  raises: [OSError], tags: [ExecIOEffect].} =
   ## Executes an external command with the current nim/nimble executable.
   ## ``Command`` must not contain the "nim " part.
   let c = selfExe() & " " & command