Throw error when reaching internal file descriptor limit

and add corresponding notes to the documentation

lib/streams.gd

@@ -649,6 +649,10 @@ DeclareOperation( "InputTextString", [ IsString ] );
 ##  handles windows-style line endings. This means it should <E>not</E> be used for
 ##  binary data. The <Ref BookName="IO" Oper="IO_File" /> function from the <Package>IO</Package>
 ##  package should be used to access binary data.
+##  <P/>
+##  Note: At most 256 files may be open for reading or writing at the same time.
+##  Use <Ref Oper="CloseStream"/> to close the input stream once you have finished
+##  reading from it.
 ##  </Description>
 ##  </ManSection>
 ##  <#/GAPDoc>
@@ -751,6 +755,10 @@ DeclareOperation( "OutputTextString", [ IsList, IsBool ] );
 ##  <C>OutputGzipFile</C> acts identically to <C>OutputTextFile</C>, except it compresses
 ##  the output with gzip.
 ##  <P/>
+##  Note: At most 256 files may be open for reading or writing at the same time.
+##  Use <Ref Oper="CloseStream"/> to close the output stream once you have finished
+##  writing to it.
+##  <P/>
 ##  <Example><![CDATA[
 ##  gap> # use a temporary directory
 ##  gap> name := Filename( DirectoryTemporary(), "test" );;

src/sysfiles.c

@@ -678,6 +678,7 @@ Int SyFopen(const Char * name, const Char * mode, BOOL transparent_compress)
 
     if ( fid == ARRAY_SIZE(syBuf) ) {
         HashUnlock(&syBuf);
+        ErrorReturnVoid("Too many open files (internal file descriptor limit reached)", 0, 0, "you can 'return;'");
         return (Int)-1;
     }